All posts would be made with hashtags like Mastodon, and then each community would just configure “Include all posts with this tag in our community”. The big issue then is who moderates tags? I think a system like Bluesky has would work well, as you mention. People can moderate tags and other people can follow their work, or not.

You’re describing something like microblogging though. That’s how that works already. We don’t need to duplicate that in theadiverse. However, just a way to see merged comment sections from different communities for the same URL would go a long way to avoid too much splintering.