Aside from the problem with the render distance of the last remaining pixel, which obviously has a big impact depending on resolution and screen size, I feel there is more.
What's the monitors resolution (fix: upscale the "pixel" (e.g. 2x2 on a 4k))
How clean is the display (fix: clean it)
Is the display set up correctly (fix, set it up correctly)
Does the display suffer from ghosting etc. (fix, buy a proper display)
Now comes the harder parts:
What's the ppi of the display (fix: super hard, you'd have to build a database of displays and take their PPI into account - low PPI displays will always have an advantage in combat and be ugly in everything else)
calculate a proper model that includes variables like size, angle, color, contrast and reflections. (fix: hard but possible: enhance the engine)
DCS lacks sense of depth and altitude. It's because it actually is in a very uncanny valley in terms of displaying real ground detail.
It's much easier/better in other simulators and it got worse on the new map technology. (fix: better details, use modern rendering technology for ground imperfections and clutter through streaming as well as optimizing rendering by making heavy use of tesselation e.g.)
This is actually a very interesting point since it doesn't just affect spotting but also the general viewing pleasure as well as massively improved gameplay for low altitude airframes.
Also it's not dependent on map size. In fact there is whole world simulations with more detail on a map than DCS but on a global scale. It's more a question of how technology is being used.
Lastly: VR users will always have it easier (not harder as many are believing) with spotting since the simulation of depth makes it easier to "unsee" the canopy.
That is because (depending on your IPD) your vision can easily blend out an object even the size of 1cm wide at a distance of up to 40-50cm. You can test this easily by holding out your finger and reading text on your screen.
(This is unfixable as flat monitors will not be able to render depth)