I'm using Fastmail for more than 9 years. Especially since they added offline support to their app, there's nothing left why I would even remotely consider leaving them.
Almost every email is going to or coming from US servers anyway. Better to treat email as insecure and use something else for more secure communication.
Correct. I mean if you send from ProtonMail to GMail (which will be 90% of private conversations) you're an open book anyway. And if you want to be sure then there's PGP et al.