Unfortunately do not use proton, but general consensus about compatibility works just fine without proton. Subdomains, to me, seem like a super easy way to identify someone - I'm going to be the only person using "abc.simplelogin.com", basically the same as if I was using my real email.
Currently, since no one is talking about Firefox Relay, I'm between either Anonaddy or Simplelogin. They seem pretty similar according to the couple comments here.