Set up a radicale server and and the calendars sync just fine. Add the cardbook adding and contacts do too. I guess thats what you mean by not first class?
AFAICT, Cardbook doesn't work with TB78 (https://gitlab.com/CardBook/CardBook/-/issues/574) . The built-in Calendar integration is real nice though - was able to pick up various CalDAV options well. Edit1: fixed link.
