As another Mudita Kompakt customer, I agree that the lack of RCS messaging is annoying, but I understand that the difficulty is that RCS currently requires Google services running. Mudita would probably have to enter into an agreement with Google to support RCS on their devices. The Mudita OS is deliberately de-Googled; this de-Googling is a security (and, perhaps, ethical) positive for many customers. It is, of course, because of this pervasive Googlisation of everything that some people want to cut Google out of their lives. The choice currently appears to be between RCS and a de-Googled device, and one can’t (currently) have both.
https://forum.mudita.com/t/issue-with-group-texts-rcs-on-mudita-kompakt/
Apparently you can tell Google to opt you out of RCS messaging via this link if your previous phone (and current number) used RCS but your current phone does not. I’ve never needed to try it: