It's hard to avoi nd this chicken and egg situation.

I use fastmail and they allow you "alias" addresses. So even though my primary email is on my custom dimain e.g. myname@mydomain.com, Fastmail also gives me an alsia of myname@fastmail.com. That's the email I used to log in (to AWS) and manage my MX records.

If either Fastmail or AWS were to go rogue/broke, I like to think I would eventually be able to restore access to my domain even without email? Hope never to find out.

Usually it’s just username/account number and password plus TOTP 2FA. Depending on country (?) they also have your postal address on file and can send you a reset password by letter. And you can specify an alternative email address like your work address, or a free email account that you only use for that purpose. Or you acquire two domains at different registrars where you can mutually use an email address at the other domain.

So just work with a decent domain registrar. Or if you have extra money, just pay the fee to become a registrar yourself.