After trying Nvidia 980, 980Ti, 1070, and 1080 cards and finding they all cause low levels of eye strain I'm now using my trusty Gigabyte G1 Gaming 970 (V1.0) card. In fact I bought a second one for the kids' PC to eliminate any issues there (again it's all good), where previously that had an AMD 6950 card (no strain on that one either).
So, as a final last-gasp I've picked up a second hand AMD Vega 64 card to try...
I've only had a couple of hours with it so far, but it feels 'uncomfortable'. Not "ouch! That's awful" just a low level sense of not being right and a quietly, slowly building headache.
I'll try it a little longer, but I feel reasonably confident that the moment I drop my 970 card back in things will feel super-comfortable again - like they should.
Barring Nvidia's new 2000 series, and AMD's older slower 5XX series, that means I've tried pretty much everything out there without success.