Dear xFra,
I have no experience with GassistPi and as your hardware is working this is goes beyond the scope of technical support, as this is an open source project we can all participate to make the ULTRA work with GassistPi.
THe good news is that I just sent a MIC+ and ULTRA+ to support the creator of GassistPi to add them to the compatible sound cards.
see here: Voice assistant