So, I've completed the game and my first culture victory rolled around right on cue between turn 249 and 250. I actually had to hold back to avoid winning too early, but I didn't expect to time it so precisely, given how I don't really know what I'm doing.
My final score is precisely 50 points, I'll post links below (no embedded pictures because it's just a boring long list - but thanks for the hint, RefSteel, I'll use it for the other images):
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I decided to only write one post about the last 100 turns, because they were, for the most part, not particularly exciting. I switched to the Merchant Republic government and noticed that Montezuma had the same form of government. Since the sum of all the modifiers was only slightly negative, I thought it would be worthwhile to try to get on his good side, and sent trade routes and made some favorable deals. But then he went and did an encore of his previous surprise attack anyway, even though I was still a bit ahead in the power ranking. He sent a couple of knights and horsemen and a few muskets to Mbamba. After a mass upgrade with the Professional Army policy, my newly promoted crossbow- and musketmen easily stopped them. Montezuma managed to pillage my Theater District, though, which was annoying. Around the same time as the Aztecs declared war, Germany made some demands which weren't too onerous, so I accepted, rather than fight a war on two fronts. Soon, Montezuma offered a peace deal, which I accepted. With my upgraded units, I was at the top of the power ranking again, and wasn't bothered by anyone for the rest of the game. Montezuma started to really like me at this time. Well, if anyone gets to hold any grudges here, it would be me, not Monty, right? And I think his hidden agenda is to suck up to leaders with large armies, so that certainly helped.
The warmongering penalty seemed to be wearing off finally, and relations with Gandhi got better too. As you can see in the screenshots below, I actually had a declaration of friendship (indicated by a blue face). The relation with Montezuma went slightly worse after a while though.
In the civics tree, I went for the Enlightenment civic, which unlocks the Rationalism policy (+ 100% beakers from Campuses), and from then on, I really left the AIs in the dust. I went on to Natural History, so I could finally fill my archeological museums. One of the special abilities of Kongo is that artifacts produce +2 food/prod and + 4 gold in addition to culture and tourism. Those yields are doubled too, when the museum is "themed" (that is, it has artifacts from three different civilizations, but the same era). I don't find this hard to achieve, since most artifacts are from the Ancient Era anyway, and artifacts can come from any of the major civs, or city states, or even barbarians. You can see the yields of themed museums in the screenshot below, (click for bigger):
It's almost like getting another commercial and industrial district in your city, and some bonus food because that's not enough! Great people kept popping up faster than I could find places to house their work. Most of them were Great Writers. It turns out that, despite what the description says, only the writer GPP are doubled...which makes sense since if you play as Kongo, you definitely don't want to build too many art museums. I still built some anyway, but nevertheless, there were many Great People waiting around. Most of them spawned at Madrid. Maybe because it had the highest population?
In particular, it's really hard to house works of music. You can only place them in your palace, or in broadcasting stations, which have only one slot, and a few wonders.
Shortly after researching the Enlightenment civic, the game really started to drag...new techs kept flashing by. Great People popped up every few turns, most of which didn't have anything to do. I built some world wonders like Oxford University, the Bolshoi Theater or the Hermitage that can house great works and grant GPP. I researched key techs and civics for the culture/tourism victory, like radio and computers. The culture victory feels horribly over-engineered, with all its slots for great works, and themed museums, and archeologist units moving around the map, and pointless little details. (Did you know that if a civ has the Enlightenment civic, your relics have less of an effect on getting tourists from them, except if you have the Christo Redentor wonder? Do you care?) I don't want to bash Civ 6, I had a lot of fun, but the late parts of the game are really meh. The AI civs are hopelessly backwards in tech, the late tech tree is boring, and so is the civic tree after you reached the most advanced governments. In my next game, I'll aim for a domination victory on a tiny map, that's for sure!
Finally, here are screenshots of my land at the end of the game (click for big)