The problem with the JMAP standard is that nothing seems to actually implement it. There is their proxy which was built in like a week and not updated and Cyrus. The latter which I couldn't (not for lack of effort) implement, and went back to dovecot which doesn't have an implementation.
Thunderbird
I really like the new Masked Email feature.
Are you worried about Fastmail being Australian based in any way?
Maybe for some, but it's never been a thought that's crossed my mind.