Now, the option to shave still does not appear on any mirror (even with beards set to growth level 3, with hair growth enabled in CAS).
I also changed the growth levels manually to be more in line with beards from the game (as it seems to me that Sims 4 Studio generally assigns lower growth levels than the ones in game - and I set them all to be >0). I've already used Sims 4 Studio's batch fix (add hair growth tags to CC Facial Hair).
However, once I try to click on a mirror in game the shave option does not appear (and hair growth is enabled in CAS). I have some pieces of CC facial hair and they work fine. Hope somebody on here might be able to help me.