Nice work on the Radxa Rock 5b. I am impressed with what you have so far.
I found couple of problems with Radxa Rock 5b with KDE Plasma.
I have a 1366x768 monitor. Actually a Samsung 32" TV. It will not display 1366x768. I do have the Compositor turned off. The screen displays a vertical green line on the left, and everything is blurred almost beyond recognition. Changed to 1920x1080 and the screen is OK.
I tried to take a screen shot with Spectacle, but spectacle captured perfectly fine. So for some reason, Spectacle saw it as OK.
In the first boot Calamares stage, RebornOS uses LXQT, however when running Calamares, LXQT is not offered as a choice.
This is the strangest of the three. When I go to the RebornOS home page with Firefox, It freezes up.
Sometimes I can use Konsole to do a
pkill Firefox
other times I have to Ctrl-ALT-F3 and manually reboot.
Other pages, such as the WiKi are OK on Radxa Rock 5b.
If I go to the RebornOS home page on my x86_64 machine, the home page is fine. It just seems to be the home page.
Full transparency, I am affiliated with EndeavourOS.
Pudge