Never heard of the issue you described, so it boots the bootloader but then eventually shows a blank blue screen, ideally would like to see a video of whats happening, so can see time scales.
Disabling CSM would more likely make it worse not better.
GPU I think is most likely culprit, if your CPU has one, can you try on integrated GPU?
Hi chrcoluk, let me see if I can get a video up to YouTube later today.
Neither of the CPUs I have on hand have an integrated GPU