Facebook does not guarantee an email address, either. (Just went through this process with Apple/Google/Facebook/Email auth - even using Firebase Auth library - surprising how many edge cases there were).
Recently went through the same debacle. Our hope was that we could solely rely on Magic Link instead of email/password ... but it turns out many users don't really understand it [1]. So your options are:
1) Email/Password Sign In
2) Bite the bullet and add Apple / the "full auth stack" (FB/Google/etc.) & deal with account linking issues.
This seems very expensive. How do you set up the monthly plan more efficiently?