I was looking at the Zwift frame and thought that could be a nice trainer solution with no derailleurs but Zwift virtual shifting is compatible with Zwift so far. That wouldn’t work for me.
If I were building a trainer bike from scratch today, I’d just find any used frame in the right size or buy an open mold Chinese frame and then fit it with a 1X crankset, and some inexpensive shifters/derailleurs. If one needed new, the mechanical Chinese groupsets (Sensah, Ltwoo) would probably be just fine for a trainer.