@alkexr I don't think you should create a bunch of different options of alliances, as that would detract from the lore of the map and I doubt you can decently balance each one. For example, I don't really like Galactic Wars with the three different alliances options, because I prefer a scenario having its own specific setup, instead of whatever. If it makes sense that it is 1 against anyone else (you know, often in fantasy there are good and bad factions), like in the case of Feudal Japan (or in a fantasy in which everyone is evil), then my suggestion would be a set of playable players for a specific number of players, I'd suggest either 3 or 4, specified in notes, leaving the rest to the AI, and the game fairly balanced around that concept.
For example, I mean you say that those 4 are the players supposed to be assigned to humans, 1 player per gamer, these players being human default, and the remaining 8 players are for AI, and being "Hard AI" default; then you try to devise and balance the game based on this scenario. Likely it would make the most sense to reserve the most civilised players for humans.
Alternatively, you can try to balance the game for 12 players, then leaving the gamers pick what they prefer, 1 player each, rest to AI, but this is going to be a mess to balance, and would oblige you to give the game an un-organic feeling, since you must take care that no player is doomed since start and, even more important, that there are no over powered players. Still, in this case, it would be advisable to write in notes a set o suggested playable nations for each number of human players, like in the case of Feudal Japan, tho I believe the suggestions therein aren't well balanced, and would not play with those myself.
Having a decent balance is very important for FFA, as most people don't enjoy playing disadvantaged players in such setups, and bidding is not really an option, with more than 2 factions.
As for the map, my main concern is that you might go too heavy with the territory effect modifiers. Very few people like to have a lot of modifiers from territory effects, and I ain't one of them.