I wonder if there's an app/extension that streamlines remembering/autofilling usernames but not passwords. I doubt many people would be into it, but it would be the best of both worlds for the case you describe, I think.

Or simply a personal allow list of origins, with a happy green indicator prominently overlaid onto login forms on those origins you've saved -- doesn't even need username storage.

Maybe even a community-sourced allow list, but that would need some seriously trusted management (including purging upon domain registration expiry/transfer) but that would mostly duplicate the domain warnings that browsers already offer, anyhow.