I believe the killer feature which solves your dillemma is that the major difference between nostr and all other social media apps out there is that nostr is NOT an app or platform, but an entire protocol. This is an important distinction as it allows both scenarios you posit, to exist. You can have everybody and a select few at the same time.

An app or platform sits on top of a protocol. Most existing social media is one app sitting on one protocol controlled by one entity.

Nostr itself is an open protocol which can have many apps/platforms/clients/etc sitting on top of it. Some of those apps can be wide open to anyone to post anything like damus or amythest. But people can also develop walled garden type apps, such as ZBD, on top of nostr.

Imagine a very specific client/app, let's call it OnlyMugs. Within the app, it only shows pics of coffee mugs. The app can sit on nostr and run its own relay that aggregates from other major relays but filters for only coffee mug pics. The rest of the world is free to continue posting whatever they want and it'll continue to be discoverable on damus, amythest, snort, iris, primal, etc etc. But if you're on OnlyMugs, you only see mugs.

Best of both worlds can happen with nostr protocol.