You can close signups on PeerTube and just run it for one channel. I’ve been doing something similar minus the YouTube mirroring. Honestly works well and is a pretty great solution. Not what you’re asking for, but it is a possible solution.

Basically, people on other activitypub services like Mastodon can follow your posts on WordPress and they’ll see them in their feed like any other content. You can install another plugin to follow others on the Fediverse from within your WordPress site as well, but I haven’t felt the need for that yet personally.

Been using this plugin for a while now, and it honestly makes it super simple to get your blog on the fediverse. Really cool, and I recommend it if your site is on WordPress. You basically have to do a tiny bit of set up, then you can pretty much forget them plugin is even there, it just works.