A router may be ~200, but I imagine monthly plans for unlimited are over 100/month from a traditional cell provider.

This seems very expensive. How do you set up the monthly plan more efficiently?

Trying to get a 100$ bill down to 50$ seems like premature optimization on van life.

I can't answer that, I'm afraid, I know very little of the Australian context, only estimated a router/antenna price based on a slightly conservative conversion of the price where I live.

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.

[1] https://snaphabit.app/blog/password-less-login/