The only solution I found is that I copied a version of the GTN plugin from a backup location back to the plugin directory. So still using the x-plane 11 version of 2022 in x-plane 12. Seems to work well. I update the Garmin stuff separately.
RXP support is dead anyhow. So no chance to get it really solved.
Would it be possible to rename the bin file to .iso or something, such that it is recognised for mounting by windows and the step to the USB driive is not required?