As Fastmail themselves have pointed out many times, e-mail is inherently a multi-party activity with both parties holding a copy of the e-mails. Aside from the microscopic amount of people GPGing their e-mail and sending it over good servers, your e-mail is as private as the other party treats it. Which isn't very much. And you can't do anything about that.
You want private and / or anonymous communications? Don't use e-mail.
The bigger issue isn't a privacy one, it's a security one. For many people, if you can get access to their email box, you can account-reset your way into any other account they own. So if an authoritarian state compromises your account (because they control the servers), there goes your everything
Fair point, but my desire is for "good enough" rather than perfect. I know my emails go into other systems, it's more about feeling that my inbox is hosted by a company solely based in a jurisdiction where it can't be compelled to do anything by the US government. It's also a matter of principle - I want to move as much of my infrastructure as I can away from US-based services because I don't trust things not to get worse there.