If you do try it again, we'd be keen for you to contact our support if you have problems so we can look into if there's anything we can do at our end.

Were you just using Fastmail as a copy if Gmail? Or were you manipulating gmail through jmap?

Gmail and Fastmail were not linked in any way. I have had my Gmail account set up in MacOS for years. I added my Fastmail account. My system became unstable and all my Internet accounts started prompting for logins.