One cause of eye strain that I've noticed is, because my external monitor doesn't automatically adjust brightness, any change in the light from outdoors can make my external monitor too bright or too dark from moment to moment.
Unfortunately I can't fully block off the large windows and I live in an area that is sunny and has clouds. Therefore, many times over the course of a day there are differences in the brightness of the screen versus the room, which can add up to a lot of eye strain. So I have to quickly adjust the brightness 10-20 times per day.
In contrast, I once had an LG monitor that I bought in 2014, that automatically adjusted the brightness or contrast from moment to moment and was no problem to look at. That said, it might have been that macOS was assisting in that, however I've generally found that it doesn't want to control the brightness of external screens.
LG seems to have more than one technology for automatically adjusting brightness/contrast e.g. DFC digital fine contrast and DCR Dynamic Contrast Ratio.
Some LG Screen have an Auto Luminance Sensor to allow automatic adjustment of brightness however they only seem to put it in medical diagnostic displays that cost $4000+.