Linux users with an external monitor and eye strain, did you try to disable Intel / AMD iGPU driver with kernel parameter? Like modprobe.blacklist=i915 for Intel or modprobe.blacklist=amdgpu for AMD. Was that helpful?
This trick applies the fallback driver, which may be more comfortable, but without many things from the manufacturer's driver, so don't expect that OpenGL / Vulkan / HW acceleration / etc. will work.
With AMD graphics cards, there can definitely be some relief. The AMD graphics cards I've encountered don't strain my eyes by default. However, when the amdgpu initializes the graphics card, I experience significant discomfort while working. In the case of the RX6600, unloading the amdgpu module provided comfort when working in Linux. With the ARC A770, I didn't notice any difference. In both cases, a true 8-bit display was used.
The amdgpu module manages a lot of models of graphics cards, including very old ones. Therefore, it's not possible to speak for all of them, as there may be exceptions in terms of some dithering algorithms, etc.
If a specific graphics card strains the eyes by default, simply unloading its native module (driver) doesn't help.
However, be aware that tearing, which occurs due to the lack of VSync, may cause slight eye strain on its own. But when working with text and scrolling, it doesn't particularly bother me. So you'll need to experiment to find what works best.