This is basically where I'm at. I love Fastmail, but going to the effort of trying to get JMAP standardized and then investing development effort to NOT put it in Thunderbird feels like there's no real intent to make email better for everyone, which is disappointing.
This is from August 2019
https://www.fastmail.com/blog/jmap-new-email-open-standard/
This announcement is a slap in our faces.