Aside from making certain types of land unacceptable for building those districts, maybe there is some trick to limit total number of districts in general or per map size.

Its very annoying to see how AI overpopulate whole territories with districts until the continent is basically just one huge city with national borders.

I was thinking that some form of hard cap for districts (expandable with tech and maybe civics and including only generic FIMSI districts?) would be a way to go, but at this point I have no idea if that would work, though Harbors being anchor points again would help making it less limiting.

I wouldn't mind if the map was more fragmented, with small towns grown around Admin Centers and various district anchors like Hamlets and main large cities around Main Plaza, but we'd really need a way to make those separated spots defensive somehow. Pollution in theory should be a limiting factor - it is in my case, I tend to keep it at the back of my head and limit myself with district spam - but it doesn't work that well on AIs and especially IPs, who quickly fill their territories up.

Soft cap that was introduced in a form of inflating Industry cost of districts was an interesting attempt, but I don't really think it works that well and it slows down the gameplay too much, especially on Endless.

I'm opposed to  a global district cap but share the displeasure about the Coruscant part of the game, you get my upvote for that part.