I’ve been using Firefox Relay for about a year and its been great. I might switch over to Simplelogin since I already pay for a premium sub with proton and its now included.
One small thing that simplelogin might do better than others is sending from an alias. Replying is fine from Firefox Relay but if I wanted to start a new email using an alias, I think simplelogin is capable but Relay isnt able to.