Did a quick search on this rather than do a new topic, I found a random reddit / forum posting from someone who said lowering the RGB monitor settings allowed them to use their new screen etc etc.
Now I have astigmatism very slightly and recently I dont know / cant remember if it was something I did but 'white' on my monitor is / was 'too white' on certain programs on my PC (GTX1660 super), no idea why. - to elaborate more text that was white (title screen) had a haze round it like a halo and it hurt to look at it, it was so bright and I would get a headache playing the game, everything 'popped' and was very vibrant. I found 'night mode' / flux cranked up helped a bit but it still was as if white was far too bright.
My RBG settings on the monitor were 50/50/50, in the end I dropped them to 30/30/30, this made all colours less dazzling for me and I could play a game I've not played without a headache in a long time for over an hour with very little discomfort.
I've always seen that certain colours are too bright for me, I use reduce white point on my ipad and this lowers colour brightness....
Using my work laptop on the same screen (same profile) was almost too dark for me, it was like looking at paper (I actually upped the brightness a bit but not the RGB)
anyway, tldr, if the screen hurts - assuming you know like mine that its 100% flicker / PWM free - try lowering the RGB settings uniformly (otherwise you'll get weird colours / all green tint etc) and this may help.