I bought a proton mail account ... but that probably presents the same problem. someday proton mail decides to shut down users. less likely I guess.

what's the least pain way to have your own domain fronting an email service provided by someone else?

All services have arbitrary usage agreements and can terminate your account at any time if some automated system is triggered, or even if a human makes the wrong call.

> what's the least pain way to have your own domain fronting an email service provided by someone else?

If you're asking about moving from Gmail to protonmail, your best bet is to set up forwarding from Gmail and go through your most important accounts to change the email.