I'm still using the XPS 9560 (Intel HD 630 / Nvidia GTX 1050 / FHD) setup mentioned in the original post without issue - it's like looking at a piece of paper for me, the best experience I've had with Windows in years. I can work on it all day in 2004 without any symptoms, using the old graphics drivers that are installed. I haven't tried updating them, and probably won't (even if it would be handy for diagnosis purposes).
However I've been experimenting with the new XPS 9500 I have just received (Intel UHD / Nvidia GTX 1650 / FHD), after updating it to 2004. Its needs more investigation but preliminary signs are not perfect.
This throws up some interesting questions, like why the XPS 9560 would give migraine symptoms in 30 minutes of use with 1909, but the same graphics hardware and drivers allow me to use 2004 all day? And is it the hardware or the drivers in new machines that are the problem, if Windows isn't?
As usual, this issue continues to baffle. Very glad I have an up-to-date, working setup though.