Proton and fastmail you can use custom domains. I only have experience using fastmail. They provide great instructions for the settings in cloudflare (mx records, etc). My domain is purchased through namecheap.

I can receive mail on *@mydomain.com and I can send email from any thing I want ad-hoc (anything@mydomain.com or anything@anything.mydomain.com)

I thought about selfhosting as well, but the internet concensus was it can be a hassle with your email getting rejected.

Is there an official community for this yet?