At which point you can setup your own relay, you don't have a block here

Unless nobody is willing to peer with you.

These are all questions we've already seen from USENET; in fact, it's not entirely clear to me what the benefit to this protocol is over USENET, except it's new.

From what I know USENET is more like Mastodon. Nostr has similarities with Mastodon but it's much more flexible, scalable, not to mention the public key infrastructure.

But IIUC, other relays don't need to peer with your relay. Your followers just need to know what relay you are using, so they can get your posts.

Correct, but that makes your node a non-distributed single-point-of-failure if the question is censorship concerns.

This sounds more and more like running a blog.