At Google, the one time I'm aware of where a Gmail SRE read the mail of a woman he was interested in, he was detected, fired, and publicly called out officially by Google, which almost never happens in this industry and probably had permanent career consequences for him. They also locked things down further after this incident.
I'd be surprised if FastMail has that level of ability and willingness to detect and react to a rogue employee.
If the privacy threat you are concerned about is Google as a company allowing your data to be used in a way you're not happy with, such as profiling you can't sufficiently control, I can't argue with your logic beyond noting that they do offer lots of controls for most of the profiling they do. I similarly can't argue if you want to avoid providers within US jurisdiction.
But if your privacy threat model includes things like "an intelligence agency wants to access your data without a legal right to demand it," "a rogue employee is hoping to access your data without an approved business need," or "foreign state-sponsored actors might want to target you," Google is one of the most private places to put your data to combat those threats.
They have the resources to address those privacy attacks, and they do so. Of course your own operational security is also relevant for each of those privacy attack vectors, regardless of which third-party email provider you use.
(Disclosure: I worked for Google 2011-2015. I never worked on Gmail, and neither work nor speak for them now.)