You've pretty much solved the issue already, I think. Try optical again but power the DAC with a USB C power adapter instead, to be sure? Both ways of powering the DAC should break the ground loop through the computer's USB power. If this works...
I'm certainly no expert on RAM timings, but aren't there settings for refresh (tREFI, maybe others?) that could need to be reduced if the frequency is reduced, to avoid too long period between refreshes? I.e. say the refresh period should be 7 µs...
So depends on the content. Good HDR content tends to have bright highlights, but not actually be a whole lot brighter total than an SDR scene, at least not for long. As an example from some analysis screenshots I took on another thread, there's a...