You seem to be concerned about two different types of snooping: internal, by Fastmail staff, and external, by government agencies.
As for internal snooping, you are correct that (some) Fastmail staff has the ability to read the contents of the emails stored on the servers. You are also correct that data is stored at rest, so if the CIA/FBI/NSA busts the door at Fastmail's data center(s), they'll get a whole lot of nothing, fast.
As for external snooping, first read this:
https://www.fastmail.com/about/privacy.html. If that doesn't put your mind at ease, then you should unfortunately sign up with someone else.