Skyname lets you register a username like example.tired.io (or another domain such as .bsky.cool) and switch your Bluesky username to it automatically. Will try and improve on communication here, I get "username registrar" is kind of hard to understand at first glance but after learning more I think it's a good description. It requires very specific user interaction to first select a new username and then register it and change your account, so logging in (make sure you generate a new App Password in settings) won't do any writes to your account automatically.