Everyone else (not just you) needs to completely delete their existing group text threads with you in them. Then you need to start a new group text yourself.
I had the same problem when I first switched to the Kompakt. The issue seems to be that both Android and iOS handle group threads in such a manner that they default to RCS if a person ever used RCS in that group at all once upon a time. So even if you remove your phone number from RCS servers and then you start a new group thread on your phone, your friends’ phones will still default to your number being treated as if it is on RCS servers, because that information is “burned” into each group thread. This is why all of your friends must completely delete their old group threads that include you.
It’s a major hassle. It’s honestly the biggest hurdle to changing from a modern smart phone to the Mudita Kompakt, in my opinion. If there were a way for the Kompakt to work with RCS, sooooooo many everyday problems would go away.