> One possibility would be to allow multiple competing groups of moderators on the same forum and everyone allowed to sign up fro whichever moderation group(s) they prefer. Then if the "official" moderators start behaving unreasonably, people will simply vote with their feet and use different ones.

That’s pretty much what Reddit is like already. If you dislike the mods of one subreddit you can join another competing subreddit, or start your own.

Likewise, with Lemmy if the people on one instance are bad, leave the instance and join another or run an instance of your own.