Hey Monie! I have created an account here just to answer you. It looks like I got the same issues as you got. Right now I'm able to use almost all Apple devices (Ipad Pro, Iphone 12, 16-inch Macbook, 2019) without any issues, the same as Nintendo Switch. Since now, I have tested a bunch of different monitors:
and all of them hurt my eyes. After a few days of using them my vision becomes blurry, and my eyes red and tired. Everything seems back to normal when I stop using them. At this point, I believe that our eyes only tolerate high pixel density screens and this is the only option for us. As for now, I'm using my Macbook with Apple Thunderbolt display. I know that this isn't a 4k monitor, but I'm able to use it with minor issues. I'm was thinking about your monitor (LG 27GN950) but it's sooo expensive.
Another option is to build an external Retina monitor, which should rather works for both of us: https://www.youtube.com/watch?v=Tdi7kB7x2Z0&t=545s