The built-in Supabase email sender is meant for testing: it allows only a couple of emails per hour for the whole project. You hit that in the first minutes.
Fastest fix (15 minutes):
- Make a free Resend (or Postmark, Brevo) account, verify your domain.
- Supabase → Authentication → Emails → SMTP Settings: enable custom SMTP, fill in their host/user/password.
- Then Authentication → Rate Limits: raise 'emails sent per hour'. It only becomes editable after custom SMTP is on, and it starts low even then.
Your temporary 'confirm email off' was a reasonable emergency move. Turn it back on once SMTP works, which you already did.