I would want to give you a word of caution: too many small groups will lead to a lot of quiet groups, it’s not fun to be in a community with 3 other people.
I’ve always been enamoured by the strategy Nextdoor took when starting out. They required 10 “founding members” to unlock a neighbourhood.
I think you could do something similar, but manual, where you unlock smaller and smaller subcommunities when membership rises.
So you’d start with maybe 5 areas for your town and only subdivide them (eventually ending up on street level?) when you reach a certain amount of members (maybe 100?).