Ever since I made this thread I have a "Machines" folder on my desktop to remind me that I should give this texture upscaling another shot. Few weeks ago I even managed to install ESRGAN like M123 suggested. I used this tutorial to install it properly, if anyone is interested:
Now I finally gave up on this idea, I think it is not worth it at current state. 256x256 is very limiting to make much of a difference and let's be honest, there is too much work and who would even care in the end.
I asked the maker of dgVoodoo about the possibility of injecting higher resolution textures using his wrapper and he said it might be possible some day in the future, so maybe then it would make more sense to work on this...