Yeah, I guess I'm confused about why an instance has any topic whatsoever associated with it.
For a federated design I'd assume the main difference among instances to be code of conduct/ToS.
I've been on one Mastodon instance and one Pleroma instance. Neither really had a set topic, and both pulled in some toots from other instances that were "similar", but my posts are about anything I want, as they would be on Twitter.
Although it might be better to say that that's the manifestation of the real problem. The real problem is that I want to be able to talk about books sometimes, even though I'm a musician so I registered on the musician instance. If there's a way for me to have conversations about books, I don't care whether it's an "instance" or not. I just don't want to manage two separate accounts. (As an end-user, none of my end-goals mention the word "instance".)