Protonmail sucks because you can’t use it with 3rd party mail clients like claws-mail or mutt without handing over $$$ (even gmail lets you do this for free, I believe). The plaintext mode in Protonmail appears not to be actually be plain text because I’ve had trouble submitting plaintext patches to the OpenBSD lists several times with it.
Have no experience with Tuta.
Self host on a VPS. OpenBSD makes it easy, follow a guide like this one.