I like the idea of nested communities. I do think the risk with trusting clients to do moderation unilaterally is that the client will also select relays, which may be authenticated. Keeping the common pattern of clients being content-agnostic and allowing users to bring their own relays seems better to me.
Discussion
similar to the wikifreedia concept, can potential forks solve alot of problems with NIP-72, like relay selection? A community can have a PR submitted for relays to be added, if this is not merged the whole community can be forked and a competing community chain is created. Could be chaos idk