Currently Thunderbird Mobile only serves as an e-mail client and it is perfectly fine. I'd suggest that instead of having an all in one app, like the Fastmail app, main features, like calendar should have a dedicated app. In practice that would mean a new, standalone calendar app from Thunderbird which could bear the Lightning brand since there were a calendar add on for Tunderbird many years ago with the same name.
It could have an icon based on the old Sunbird icon but with the same style as the current Thunderbird icon.
It would be really nice if that would be a thing for the arrival of Thundermail which will have a calendar service, and even Appointments feature could have a place in this app too.
Also on Android de-Googling isn't an easy task since Android phones relies on Google apps heavily. Using Google Calendar for Thundermail would defeat the whole purpose of avoiding Google by using Thundermail.
Yes, there is Fossify and other apps exists too, but Thunderbird Lightning could bring a better UX than these apps.