I experienced the same issue with the Alienware 27" 360 Hz OLED monitor. While it was great for gaming and I loved the refresh rate, it was terrible for everything else. Text always looked blurry, my eyes would keep trying to focus it and tired themselves out. I started getting headaches and began to doubt my prescription. After months of investigation, visits to the optometrist, I finally narrowed it down to the monitor being the culprit. I gave it away to my son, who on the other hand was thrilled to receive a bonafide gaming monitor. He uses it at 2x scaling and couldn't be happier.
These monitors are great when scaled but have real issues rendering text and other fine details at their native resolution.
Given that there isn't any objective basis for the default scaling factor of a monitor being what we call 1x, I would argue that 2x scaling isn't any less native than 1x
The 2x scaling refers to the OS UI being set to 200%. That means each UI pixel is rendered by 2x2 physical pixels of the monitor. It has a very clear and objective definition. Did you mean something else?
I think it's definitely a clear definition of what you're doing, but I still take some issue with the framing of "UI pixels". I would argue that it's more like abstract "UI distance units."
Most UI's are designed to be "right-sized" for humans with a "distance unit" being about 1/96th of an inch (when the display is the designer-expected distance from the user). If you deviate too far from this, things end up being really big or really small and the display is harder to use. What you're effectively doing when you enable "2x scaling" is setting the "abstract distance unit" that gets applied to UIs to be the size of two display pixels, rather than one display pixel. Whether this is a good thing to do and is more or less "native", IMO, depends on the DPI of the display.
These monitors are great when scaled but have real issues rendering text and other fine details at their native resolution.