agree that login with nostr offers a lot of wins!
we implemented "Login With Nostr" on https://vida.page via a simple 2-Factor DM flow. it's a UX users are familiar with and works on web+mobile.
basically the user gives us a pubkey or a nip-05. if nip-05 we check for relays and auto fill, if just pubkey we also ask them for a relay. then we send them a pin via encrypted DM and ask them to input the pin. if successful, we will their public details via the same relay and provision their account.