Hello again,
I've been using ReShade with Quake III Arena, or more precisely, with the excessive dawn source port (edawn-mod.org/).
The problem is that ReShade only applies to a part of the DSR resolution, 1920x1080 in 4K, and the same in 5K (which makes it a little less than a quarter).
opengl32.dll does not get loaded with the unmodified (GOG version) Quake III Arena, so I cannot make a comparison with that..