NIP-29 is literally titled "Relay based groups". I get that your concept here is likely a superset. A community can exist across multiple relays and based on the relay the user is looking at, they'll see a different version of the community content.

We'll leave aside the challenges of building a client that doesn't make that really hard to understand as a user (e.g. you might see replies to original posts that you don't see, etc) as I think that's ultimately solvable.

How do you see private groups fitting in here? Is your concept focused on public content only?

I really like the ideas, it's definitely a bit of a different model that would have the chance of working really well as a reddit style app but some of the magic of the single feed type apps is that you're always discovering something new or seeing things that are outside (but maybe adjacent) to the areas that you've signaled interest in. Separating into communities closes that discovery pathway quite a lot.