I decided to take one final Hail Mary at this as the few remaining quirks of the Xorg drivers were getting a little frustrating.
After weeks of hair-tearing frustration the fix was trivially, trivially easy - I changed the ‘Brightness’ level in the relevant monitor’s ‘Color Correction’ tab in the Nvidia settings, then I changed it back. It was that damned simple.
It doesn’t, however, work flawlessly. The problem always appears on a restart. Additionally, the slider/value will often need to be changed multiple times before you ‘luck out’ and the flickering stops (I’m glad it worked without issue the very first time I tried it, or I’d have given it up as a lost cause).
EDIT: Right, I have a guaranteed fix! Changing the ‘Dithering Mode’ in the ‘Controls’ tab to ‘Static’ completely eliminates the issue!
Now that I’ve confirmed it is 100% a software issue (not that I had any doubt) may I ask, again, where to properly submit bug reports? There’s quite obviously no point submitting them in this forum.