Hi :)
How concernt are you about leaking your personal information by using a custom-domain for emails?
Because anyone could see, who registered the domain, correct?
You can use Njalla to register domains anonymously. They offer anonymous crypto currency payments with currencies like Monero. The company was created by one of the founders of The Pirate Bay. They're also on Mastodon: https://njalla.social/@njalla
Most have WHOIS privacy. Anybody looking up the domain can see that it's registered to whichever company you choose but cannot get your direct information from it. Obviously, if you're under any kind of investigation or something, then the government will just subpoena the domain registrar and ask for your personal information.