You're basically suggesting bluesky style label services, except as the only solution
And no that can not be the only solution avaliable, ESPECIALLY not in communities around important topics like security, health, or for marginalized communities, etc. Your suggested default would be a trashfire by default until people have opted into some kind of moderation filters. And few will review the filters they subscribe to.
You also haven't solved the issue of how to get people to submit content to smaller communities
You have probably never seen a well moderated community, or at least not participated in one for long.
See also https://slrpnk.net/comment/10312933
What you're suggesting can't work
I sympathize with some of it, but you're going too far
Content addressable posts like what Bluesky's atproto does and cryptographic identity allows for portable posts and identities, and it even allows forkable communities as you can import and move entire conversations, and even mirror conversations that one team of mods may not like into another community (I made my first blog post about content addressable forums literally a whole decade ago)
Literally impossible according to the CAP theorem (database terminology) in a decentralized network where not all servers federate with all (often because they just never have interacted and thus don't know of each other)
You have to push the communities to participate in multiple parallel communities, that's much more reliable. Together with a credible threat that the community can depose bad mod teams by forking, you have a much better chance of preventing bad mod behavior