Yes, the change from “Legacy Support Enable and Secure Boot Disable” to “Legacy Support Disable and Secure Boot Disable” has solved the problem. I was able to boot into KDE Plasma (not sure if X11 or Wayland, TBH) 3 times in a row.
I always thought “Legacy Support” affects the way USB boot media is handled - GPT/MBR, or something like that. I would never imagine it affects iGPU / APU. Go figure. 