I'll start by just commenting on things you may have missed:
- You can opt out of search indexing in settings (and this interoperates with a similar setting on Mastodon, and possibly other apps)
- The create profile form asks for a name/pseudonym and a username, with a preview like you suggested. It does pre-fill the username field based on the name you enter, which you can then edit, maybe that feature is actually not helpful?