I have no problem with Tuta over Tor, but I'm on a paid plan (you can pay via XMR for anonymity). The issue that the tota.com domain gets blocked by providers can only be solved by using your own domain (bought at njal.la via XMR is a cheap way, but you technically won't own the domain then). Setup for own domains in tuta is easy and well documented.
I've heard that proton won't let you easily register an account over Tor (they'll ask for verification and/or block you shortly after), but I can't speak from experience. You may want to try mailbox.org as they are the only other recommended service by privacyguides, in case you value their opinion.
Good luck, and remember that emails are never really private or anonymous and should be avoided whenever better methods are available.