One thing I might be interested in is the "contacts" side of mail. In an effort to move away from too much Google, I ditched Google Contacts and host my own CardDAV using "Radicale" [0]. This works, and I also access it via DAVx on Android.
But would a CardDAV server be something worthwhile via Thundermail? Or perhaps too small a service itself? Maybe part of their scheduling tool "Appointment"? I might prefer having this hosted elsewhere than in my house.
https://www.fastmail.help/hc/en-us/articles/1500000278342-Se...