AVE10N
Is there anything in the variable names related to NVIDIA or dithering? If not, then the problem is very unlikely to be in UEFI storage. There is still a chance that something was written to an NVRAM variable that was not created by the NVIDIA driver, but this is even less likely.
GPU drivers normally store settings in the Windows registry, so they do not survive a full OS reinstall. Different operating systems and monitors can also have different dithering and bit-depth settings.
Even if GPUs had editable NVRAM, it wouldn’t explain why both cards now have issues when only one was modified.
The next step is to isolate the hardware causing the problem: test each GPU on another motherboard or try a different power supply. However, it is unclear whether unstable power can cause visual artifacts.
But the strain on my eyes remained even in Ubuntu, so it's definitely not a driver or Windows issue.
In Linux, dithering may be enabled by default on your GPU.