I went to play a SP Kongo myself to get some feel for it, some analysis by me. For you who played a lot Kongo this might be repetitive but I give it a shot.
Leader's ability: Religious Convert. No Holy sites, GP and found religion, gain all beliefs of religion majority in cities and gain apostle when theater or Mbanza if that city got majority religion.
Consequence: No Holy Site, at first look you can skip Astrology, but it leads to some useful upper part of tech three, including Cartography which unlocks Caravel and two Caravels give inspiration for Exploration which unlocks Merchant Republic, medieval government most suitable for Kongo.
So not skip Astrology but do it at later point together with Sailing (I might fog gaze sea or lake north, turn on settler map to see how many housing the most northern tiles gives, if 3 then it is sea, if 5 then it is a lake). Sailing don't give eureka when settling at lake, only at sea. Then Celestial Navigation (2 sea resources) to unlock harbors, Kongo favors commercial districts over harbors, but when possible put harbor adjacent commercial districts. Shipbuilding (2 ships) leads to Cartography (2 harbors) to unlock 2 Caravels needed for Exploration.
-In short, don't go after upper tech tree asap but do it at later point when you thinks that you isn't far from researching Exploration, around time that you just finished researching Guilds I think. If you need to get some techs from upper path earlier for example because presence of sea or lakes with seafood, feel free to do it.
Hidden cost of no Holy Site is it becomes bit harder to get eureka for Mathematics and inspiration for Civil Engineering with one district less. However that shouldn't a problem, just campus, commercial district and theater for Mathematics. For Civil Engineering problem is actually solved by Mbanza which unlike neighborhood is located at Guilds instead Urbanization which is located after Civil Engineering.
No GP mean no build Stonehenge, Kongo can do it but it leads to nowhere unless Kongo already got majority religion, in that case Stonehenge creates one apostle of that majority religion (same happen if a player who founded religion already builds Stonehenge, result: just a free apostle).
No found religion, this is tiny map with 4 players, so total available religions for all players are 4/2 + 1 = 3 religions. With Kongo here, other 3 players are 100% guaranteed of religion, only question is when to found it.
Gain all beliefs of religion majority in cities. Check which player got DotF belief and welcome his missionaries/apostles with open arms. When creating theater or Mbanza, get apostle and spread DotF over of your cities.
There is a risk however, you can't really stop religious victory of that player if he is close of it. Also it mean no allying with that player against other two since eliminating other two means a religious victory for that player. Best scenario: that player with DotF get eliminated and you still maintain his religion in you cities and with apostles you can protect your devout people against blasphemers of other religions.
I already spoke of consequence for tech tree: Kongo can delay upper part of tech tree. What of civic tree? My answer: completely ignore following civics: Theology, Divine Right and Reformed Church. They requires religion related inspirations and Kongo shall make little use of policy cards, governments and wonders related with them. It is just waste of culture to go after those three civics and instead spend culture at beeline to Guilds and Natural History afterwards. Chivalry is policy card for faster knights, but it is a lot better to mass build heavy chariots with maneuver policy card before finishing Stirrups to unlock Knights. Wonder Mont St.Michel gives apostles martyr promotion for relics but it requires a theological combat and hostile players are more likely to finish apostle off with military unit to prevent martyrdom.
One civic I didn't mention is Mysticism, because it is okay to pick up it. Kongo still can found Pantheon, so inspiration for it and it is one of earliest envoy available. Also +2 Great Scientist per turn policy card (Inspiration) is located at Mysticism, finally Oracle is interesting wonder for Kongo: faster GM and GWAM. Also Oracle gives discount for purchasing Great People with faith and Kongo might produce some faith and no idea what to do with them, Oracle can solve that little problem. I am not saying now that we should go for Oracle asap, but keep it in mind of its usefulness.
Unique Unit: Ngao Mbemba.
I don't have any new to say here, more expensive to upgrade into Ngao from warriors but cheaper to upgrade into Musketman afterwards. Good against ranged units, moves in forest/rainforest if they are normal tiles.
Seeing through forests while others don't might give some opportunity to steal builders or even better settlers. No iron needed but needs Iron Working to unlock them, benefits from classical GG and Oligarchy.
Unique District: Mbanza.
It comes with Guild and always gives 5 housing, 2 food and 4 gold. The catch is that it can only be built on forest or rainforest. I have one question that I never tested it and you maybe do: at Conservation civic (after Natural History civic) builders are allowed to plant woods, can we put Mbenza there afterwards?
When test playing Kongo I find myself often placing Mbenza at 2

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flat rainforest tile which is one of most useless tiles in civilization 6, the one where settler is standing now is perfect candidate for Mbanza. 2

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rainforest is located at plain hills, so I always chop and place mine at it.
Normal forests often fell victim to my chopping due better production yield from forests compared with rainforest. Riverside forests get lumber milled, I advice to do that with forest NW of dye tile this PBEM. Forest SE of dye tile is not along river, I generally chop and mine such tiles.
That 2

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forest tile 2 tiles SW of dye, that is interesting subject for discussion. After chop what use for it, farms? Or maybe better preserve that forest for Mbanza? That is decisions for future.
To unlock it, need to research Guilds. So beeline for it with possible backtracking for some useful civics. In ancient age usual route: Code of Laws - Craftsmanship/Foreign Trade - Early Empire/State Workforce to end at Political Philosophy.
After Political Philosophy well known Drama&Poetry and Games&Recreation bottleneck since most players struggle to get inspiration for both of them while maintaining good growth curve, often resulting in skipping inspiration and researching it longer. This might be good moment to finish Mysticism and Military Tradition if you hasn't done it earlier.
After those bottleneck, Recorded History and Defensive Tactics and then advance to medieval age with Feudalism and Civil Service to finally unlock Guilds.
So to get whole picture: Code of Laws - Craftsmanship/Foreign Trade - Early Empire/State Workforce - Political Philosophy - Drama&Poetry/Games&Recreation - Recorded History/Defensive Tactics - Feudalism/Civil Service - Guilds.
Inspirations are follows: Improve 3 tiles/find second continent - reach 6 population in whole empire/finish a district - find 3 city states - finish a wonder/finish construction tech - have 2 campuses/get DoW'ed by other player - have 6 farms/reach 10 population in one city - create 2 markets.
So we need at least: 2 campuses, 2 commercial districts with 2 markets, a city with population of 10 supported by farms. Wonder and construction are hardest to make it in before we finish civic, getting DoW'ed by other player is outside our control.
However no need for a real hard beeline, also pick up Mysticism, Military Tradition, Military Training and Mercenaries along the way if situation asks for it. (Found Pantheon, raze barb camp, build a encampment and have 8 units).
Regarding Games&Recreation bottleneck, it need have construction finished and construction needs watermill. Our route to construction looks good: need masonry (quarry) and horseback riding (create a pasture) and riverside capital, so watermill available to build. Problem here is wheel which unlocks watermill, need to find copper or other mine-able resource nearby or research Bronze Working and pray that iron resource is nearby.
Drama&Poetry is probably the one that we don't want delay too much since it unlocks theater district. So either full research it or attempt for a wonder which can finished relative fast. There are 4 fairly early vanilla wonders: Stonehenge which shall be ignored by Kongo, Hanging Gardens is cheapest together with Stonehenge, we can build it riverside and keep future theater district in mind, but extra housing and growth is bit useless for Kongo since Mbanza itself is actually better than Hanging Gardens. Then two bit more pricey wonders compared with first two: Pyramids and Oracle. Both are good for Kongo and I think around turn 40 we need to decide whether to go for one of those two seriously or otherwise just full research Drama and Poetry soon after Political Philosophy is finished.
Now at last subject:
Civilization Ability: Nkisi
Palace get 5 Great Work slots, no need have first Great Writer wait till amphitheater getting finished to create 2nd Great Work. 50% faster generation of GWAM and Great Merchant. 2

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from every relic, artifact and sculpture from Great Artist in addition of culture and tourism.
Together with Mbanza it creates powerful engine which Kongo shall ride upon to victory, the catch is how to collect them.
Relic: no huts at PBEM's so no chance to randomly pick it up. Mont St.Micheal wonder gives apostles martyr promotion, but as I said against players it is going be tricky compared with AI. You can attempt to have theological battle while DoF is in place and provoke opponent with spreading if he isn't sending apostles/inquisitors to deal with your apostle. But a clever player just allow you to spread religion, forcing you to run out of charges and when apostle's charges run out, they disappear without giving relic. Opponent can just wait till DoF ends and then kill your apostle with military unit. Also Kongo's apostles seem to have choice from 3 promotions instead normal two for all other civilization's apostles, this increases chance of martyr appearing as one of choices for apostle's promotion.
Finally maybe there is Kandy at this map, if it is then great! Don't scout further in that case and wait till suzeraining Kandy and go on natural wonder hunt with scouts. Every PBEM map get always 3 natural wonder due world builder crashing if you attempt to move, add or remove it. Maximum 3 relics in that case if we don't accidentally stumble upon natural wonder already before meeting Kandy.
Great Artist's sculpture is not really good, mainly due randomness of it. In worst case just a painter who creates 3 portrait, landscapes or religious works. First Great Artist is a Renaissance one, when starting game, check Great People screenshot to see who is available, if it is Donatello, Jackpot! Michelangelo is okay second option. But after that, industrial and modern GA are trash, Atomic age got two GA with three sculptures, information age one with three sculptures. If one of opponents happens to claim GA with sculptures, we can attempt to make trade, or demand it with peace treaty when we trashed that opponent with our army or send our spy to "borrow" their sculptures.
Lastly and best way for Kongo to make use of Nkisi: artifacts. Need to unlock archaeological museum with Humanism (get Great Artist) and Natural History (build a archaeological museum) for Archaeologist.
So beeline for Natural History thankfully is same as for Guilds, after Guilds Medieval Faires (4 trade routes, so 3 commercial districts or 2 commercial district and one harbor), Humanism (get Great Artist, peanuts for Kongo normally), Mercantilism (get Great Merchant, also peanuts for Kongo normally), Colonialism (research Astronomy, aha here is our bottleneck), then finally Natural History (build a archaeological museum).
EDIT: oh a suggestion here: build Terracotta Army wonder which unlocks with construction before you finishes Natural History, this way you archaeologists can enter borders of other players without asking them to open them.
Aside this beeline we should go for Exploration too for Merchant Republic government, as stated earlier start with beeline towards cartography when Guilds is finished for unlocking Caravels.
Also unlock Enlightenment (3 Great People, should be peanuts for Kongo, right?) for powerful policy cards as rationalism, need to unlock diplomatic service before it, maybe one of players is willing to create alliance.
Almost all inspirations should go naturally for Kongo except one for Colonialism which requires Astronomy tech, well then a beeline for Astronomy here:
Animal Husbandry/Pottery - Writing/Archery - Horseback Riding/Currency - Mathematics/Apprenticeship - Education - Astronomy.
Eureka's: meet with other player/kill unit with slinger - improve a pasture/create trade route - create 3 different districts/create 3 mines - grab Great Scienist - build university at campus adjacent mountain.
Things start get interesting with mathematics, we need 3 different districts, so as said before already we should already get campus (2x for recorded history inspiration) and commercial district (2x for Guilds), then additional theater maybe or encampment due possible aggression of other players.
Education requires a GS,
so I propose here: first district that we shall build is campus to claim first GS. Moreover it shall be adjacent mountain since we are already seeing a mountain at east to gain eureka for Astronomy if we finish university there.
This tech beeline follows center greatly, we shall go for upper tech tree after finishing Guilds. The bottom tech tree however is filled with military tech and we need it for our safety or mischievous ideas with opponents.
Wheel, Bronze Working and Masonry are already needed for construction to deal with Games&Recreation bottleneck. Iron Working of course for Ngao, Engineering because it leads to machinery which unlocks crossbowman, it unlocks aqueduct which is eureka for military engineering and also lumber mills. Castles can be ignored till Astronomy is finished. Stirrups, Military Engineering and Gunpowder are too important for defense, no ignoring here.
What after grabbing Natural History? Conservation for possible planting woods by builders for Mbanza, to get inspiration for it just place a Mbenza at a breathtaking tile if it appears.
Though due this late it is more likely to go for Civil Engineering first for improved version of builders or Nationalism for war.
Phew that is long text from me, sorry for bombarding you with ideas.