No, I do not have troubles with smartphones. I used to have a Motorola G2, now ‒ an iPhone SE, and none of them caused any discomfort. iPad 2018 is also fine: I can spend hours with it.
I did not update this thread recently as I'm experimenting and trying to find the solutions.
I installed BootCamp and I had no issues at all on the same iMac. The default 10-bit color in Windows is ok to my eyes. I only had an impression that the same display in Windows looks "simpler" than in MacOS. Colors in MacOS are much more saturated. To be honest, I wish I could make the black color not so deep.
I also found out that in MacOS I only have troubles with the text. The movies or YouTube are fine.
I'm trying different things: changing the brightness, changing resolution in MacOS itself and in Windows under Parallels Desktop, but I do not have any sure results yet. It only seems that non-integer scaling is not good. I'm now back to the native resolution 2560x1440 and I switched scaling in Parallels from "Best for retina displays" to "Best for external displays", which also seems better for my eyes. So far, no dizziness, no pains, but my eyes seem to get tired faster than on other screens.