Thanks for the suggestions. Probably have a core20 version of the kde framework and kde-neon extension is the most promising here, hopfefully with ARM support someday. At least I think the extension gives currently the best experience for packagers in regards of using it.
I also saw this very similar thread:
Same issue: How to get a newer version of Qt5. There it is suggested to use a PPA, maybe another thing to try, but seems to depend on a preview feature of snapcraft.
Currently I try to get this to work using the deprecated desktop helpers using the core20, but the resulting build is giving me other headaches :(.
As I am dealing with only a single application that’s about the same as bundling this into the snap.