I think the key question I'd have is the one you are still figuring out the answer: given a deployment of a hubzilla server with a configuration, how many users can you serve it?
E.g, how small is the $6/month "small instance"? Can it work with 10 users? 100? 3? And what happens when the average user follows 100 other people on the Fediverse? And what happens if one of these users follows a bunch of media heavy content? What would happen if one popular user is followed by 2M people?