I solved the problem by setting a different color profile for the Dell s2416h monitor. It is exposed, as on Windows, sRGB IEC61966.

I have exactly the same problem right now!
I switched to a MacBook, worked without an external monitor. My eyes just fall off in the evening. In fact, this problem is common, Google it. Someone's eyes are responding normally.
I visited an optometrist, tried drops, glasses, eye exercises, breaks, bought an external separate monitor.
I turned off font smoothing and my eyes are about 50% less strained now.
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