

I’d highly recommend getting your own domain.
Make sure it’s something that’s cheap to renew, cloudfare registrar prices should be at wholesale price.
Setting it up correctly can be a pain, but once your privacy friendly provider starts getting inevitably enshitiffied, you can move your email way more easily.
I think so, at last. Never had to move from Proton yet.

Interesting, it might’ve changed, I’ve had my domain for 5+ years by now, but if you set up all the signing keys, DKIM, SPF, and whatever properly, I never had a single issue as far as I know.
But I never tried sending my own emails - I’ve been using Protonmail’s custom domain option, which I’m sure helps. Running your own mailserver without a trusted provider might be a lot more problematic, I never even considered it. The idea wasn’t to run my own mailserver, but to be able to move my custom domain to a different provider that supports custom domains, so I don’t have to change my email anywhere.
I just made sure that my domain is passing all the email domain checker tests, and as long as I don’t get on a blacklist, I should be fine.