I didn't quite understand the ID thing. Is this number 10 for 8 factions, plus 2 for neutrals and the "game-breaking element in the construction window"? So, if you create a new faction, you need to assign it an ID of 11, but the game doesn't allow that? (Is it the game engine, the modding program, or something else?)

I'm not an expert in code, I don't know all the nuances.
You can add an ID – there are already mods for artifacts and creatures that do this. The question is how to make these IDs work.
If you add an ID, you'll get another neutral city – you go into the construction menu, and the interface disappears because the city doesn't have a construction scheme. It's unclear whether the scheme is tied to the ID in the code or if there's a list of these scheme files somewhere. It's also unclear how to make the faction playable, how to give it icons for captured buildings, how to link buildings and building effects to it...