Thanks for all the replies. With respect to binocular vision dysfunction, I can't say but it's clear that I have a severe sensitivity to whatever is happening on this particular computer. I will review the link about the COVD optometrist. Again, I'm a technology trainer who has worked in both broadcasting and training environments my entire career, staring at all manner of television and computer displays for hours, days and weeks. The only two times I've had this type of reaction is on this iMac Pro when it's doing whatever it's doing, and when purchasing a 55" LED HDTV over a decade ago. The eye strain, fatigue and nausea was quite intense in both of those scenarios. The LED HDTV was exchanged for a Plasma TV which eliminated that issue. I'm fairly certain that was related to frame rate playback on that type of screen. Whatever is going on with the iMac Pro obviously can be "masked" or shutoff if I can figure out how again. I do have Time Machine backups and when the problem first occurred about two years ago the first thing I did was wipe and restore the iMac from a backup prior to the the problem. It didn't help. Apparently whatever is causing the issue is not impacted by restoration of earlier configurations, or isn't resolved by a restoration. I've also tried booting into Safe Mode. That didn't help. Trying to find a history of my terminal commands is interesting and calling Apple Accessibility on that point might be useful. Again, this would go back to somewhere between 1.5 - 2 years when I would have run the command that resolved the issue and I don't know if any terminal history would go back that far but I'll explore that. It's a good suggestion.
The problem with figuring out how I resolved it the first time is that I do remember I was trying every conceivable solution one after another, sometimes mixing them one after another (i.e. a terminal command, followed by some changes in SwitchResX, followed by rebooting the computer which is frequently required to make these types of changes actually take affect. That's why it's so difficult to figure out which change actually was the one that resolved the issue the first time around. It might have been a combo of actions one after another that I didn't see the result of until I rebooted. What's interesting is that the very first time I booted the computer when taking it out of the box new, and the only thing on the screen was the white Apple logo on the black background, I remember instantly feeling the "pain"/eye strain. It was brighter than what I'm used to. Having responsibility for literally dozens of Mac MBP's, iMacs and PCs, I knew something different was going on this time. I thought it was potentially just a result of this being one of the first iMac Pro's and my first Retina screen in my home. I had a standard iMac for about a decade prior to that….no similar issues. BTW - I'm realizing I didn't purchase the iMac Pro in 2019…it was one of the first which I purchased in 2017. I did spend hours on the phone with Apple Care trying to resolve it but they couldn't help. Once I did fall across the solution, I knew I had done so even before the machine fully rebooted because when it was coming back up, that white Apple log seemed normal brightness to me.
I also own a 15" MBP that I did purchase in 2019. Interesting that I have never had a problem with it's screen, although the last few days as I'm using it going back and forth between the iMac Pro, I am feeing similar symptoms. I can't be sure if it's not just that I'm hurting my eyes from looking at the iMac Pro so they're already weakened as I'm looking at the MBP. My gut is telling me the iMac Pro issue has something to do with more tightly packed pixels on the retina screen. I have tried lower resolutions, non-HiDPI resolutions, using terminal commands to shut off font smoothing and dithering. None of it is working. Could it be a recent software update? I suppose I can't rule anything out. But again, the problem didn't return immediately after any update. It returned the instant I tried to scrutinize the exact screen resolution in the System Preferences > Display. I'e also owned iPhones since the beginning including my iPhone 12 Pro Max. No issues on those either.
I am seriously thinking about acquiring another computer but two issues are slowing me down: #1 I know this is fixable if I can remember how and it seems wasteful to push aside a machine I paid over $5000 for a few years ago which is still great in terms of all other functionality #2 How can I determine if any other new computer won't give me a similar issue? I'm a Mac guy who also works in Windows environments, and I can't see myself giving up on Macs. But I'm not sure even purchasing something like a Mac-mini or Mac Studio or Mac Pro with an external monitor wouldn't give me the same problem at this point.
Again, I appreciate the input.