I understand there was a bug in v2.12.3. I had these same problems, really bad.
Devialet released a fix today, v2.12.5 which addressed these issues. I installed it this morning and everything seems to be working fine now. My Hifiberry HAT/RPi is now out of the system and Roon Ready mode is working nicely.
I can’t address the48 Khz limitation, but it has been discussed widely over at the Devialet Chat forum.