I’m not sure how this would work, but what about the concept of cross-instance communities? For users it would be a bit like a multi-reddit where you group various communities together into one aggregate list but when posting content you’d have to choose which instance it lands on. Mods would have to agree on a set of rules (and you’d have some communities split off due to differences), but otherwise it seems somewhat plausible.
That would be one way to solve the problem of every instance having a version of one specific type of community.