Install it and use it?

Their PDS is self hosted, but it does still rely on the central relays (though you COULD host that yourself if you wanted to pay for it, I suppose?).

It's very centralized, but it's not that different from what you'd have to do to make Mastodon useful: a small/single user instance will get zero content, even if you follow a lot of people, without also adding several relays to work around some of the design decisions made by the Mastodon team regarding replies and how federation works for those kind of things, as well as to populate hashtags and searches and such.

Though really you shouldn't do any of that, and just use a good platform for discussion, like a forum or a threadiverse platform. (No seriously, absolutely hate "microblog" shit because it's designed to just be zingers and hot takes and not actual meaningful conversations.)