Assuming that you trust what Proton says, when they receive a (possibly unencrypted) message they re-encrypt it with your key as soon as possible and they don’t log the content. So, after that point, they (or anyone else) can’t read the email contents. If it was also encrypted in transit, then there’s only a small window inside their email processing system where the plaintext was passed from one encryption to the other. It’s only decrypted again in your browser or proton mail app with the key that only you have. It’s not bulletproof, but it’s better than most providers.
Your domain name could be ordered to be removed from US-based dns providers, no matter which TLD it is. That would essentially block your website from most US-based viewers without actually shutting down your hosting. Advanced users could still get to it, though. Consider hosting through Tor and a .onion address for more resiliency.
Look into https://simplelogin.io/
They make creating random aliases for custom domains like this easy.
As for the domain name itself, anything that already looks like a mail service is good. “examplemail.com” or “mailexample.com”
Tailscale (https://tailscale.com/) works great for remote access to your private services. Once the wireguard tunnel is established, then the traffic is peer-to-peer (assuming it’s configured correctly) and not through their centralized servers. Even from a mobile device.
You might enjoy reading Extreme Privacy by Michael Bazzell
U.S. Immigration and Customs Enforcement www.ice.gov
https://simplelogin.io/ (owned by Proton) is great for this. They have a feature to generate an email address by random word or even by uuid.
It’s probably too complex for what you’re looking for, but https://www.kannel.org/ is a large-scale SMS gateway.
And they keep making it harder and harder to not use a Microsoft Account.
https://www.techradar.com/computing/windows/microsoft-is-removing-known-mechanisms-for-creating-a-local-account