Honestly, that’s not a terrible idea. There are only a half dozen accounts which actually matter, so there is not even that much initial configuration burden. If I get phished for my HN account, oh well.

Think my only blocker would be if the browser extension fights me if I try to register a site using a broken/missing password.

Does feel like a bit of a browser gap. “You have previously visited this site N times”. If that number is zero, extra caution warranted. Even just a bit of extra sophistication on bookmarks if the root domain has previously been registered. Thinking out loud, I guess I could just lean on the browser Saved Passwords list. I’ve never been comfortable with the security, but I could just always try to get it to save a sentinel username, “YOUHAVEBEENHEREBEFORE”.