I've been testing on FHD, UHD, even 8k. It does all look the same on big, expensive tv's.
My suggestion is going the opposite route with Diablo and turning off ALL of the graphics enhancements on the cpu and going with a custom 1280x960 resolution, or 1280x1024 if its not an option, and in the Diablo GoG Launcher select 640x480 60hz / Fullscreen - Integer Scaling. You can refer to my post from 4/1/19 for an exact readout for the options.
GoG suggests the Integer Scaling of 640x480 for the 1280x960 resolution because, AND I QUOTE:
"If your display supports 1280x960 resolution,or any other multiple of 640x480, you can have a crisp image devoid of hideous bilinear interpolation with a few simple configuration tweaks. This kind of scaling is called integer scaling and crudely doubles the image--but we like that. Especially for old games. This way there is zero aliasing.
Most of us have 1920x1080 displays, and even if we preserve aspect ratio, an internal 640x480 resolution does not scale up neatly to 1920x1080. With a 4:3 game (like Archimedean Dynasty), you end up with an ugly, filtered 1440x1080 image, (1920x1080 with black borders). If you're willing to give up a couple pixels of screen coverage, you can have a crisp doubled resolution of 1280x960." -www.gog.com/forum/archimedean_dynasty/integer_scaling_the_key_to_a_sharp_fullscreen_image