New posts should be in a general, catch-all section. OP could suggest a community/tag but it wouldn’t appear there right away. Other users can tag the post for a particular community or label it SPAM/Troll/Abuse.

If enough users tag a post for a community, it shows up in that community feed.

New users can’t tag until they reach a threshold of comments/posts that are positively received—not SPAM, has upvotes, replies.

This will enforce a level of moderation even when the community mods have ghosted.

There would be no cross-posting per se. Posts could be tagged for multiple communities but must reach a certain amount of tags for each community before it appears there.