Yea, I have been wrestling with this same issue for the past whole year.
It is so puzzling, and every time I think I have a solution, the headache/nausea/mental disturbance once again comes back.
I even bought an EZ pro 24" blue light shield for the monitor and would still get the headache.
I even tried using it with an old desktop with no dithering, still the same issue.
The only thing left is the flicker (although it says flicker free). Something to do with the hz of flicker?
I can use my LED 17.3" Asus G752VT laptop and Dell XPS 13 9350 LED laptop though with almost no issues.
I wonder what is different in the monitor that is making it unusable.
Earlier I had a 4k LG 27" IPS monitor, then I replaced that with a Samsung FHD 24" monitor with only 250nits brightness, and yet the exact same issue is still there.
I thought vision therapy might resolve it, but you mention that even that only helps but does not solve the core issue.
The biggest elephant in the room is that how is nobody else complaining? I mean it seems like the people with issues are like 1% or less. Are the rest of the people really not having any issues?
If I cannot use a monitor at all, I'm stuck to using only laptops.
I'm wondering if a VA or TN might solve this.
I have seen these 2 models mentioned often in the forum: LG 32GK650 VA and LG24600F TN.