Using the same ESRGAN program that I was testing to upscale the textures it is possible to remove dithering and color banding in original textures and artwork, without changing resolution or changing the image in some more aggressive way. Using the "DeDither" model It gives the images it's original "uncompressed" look.

In this artwork it removed the pixelisation:
maggsx_rlt_dd.png
Untitled.png

Here you can see on the right the transition between areas of different color is much smoother. Even though it is same resolution, the image is much higher quality:
Untitled2.png

Here you can see both:
Untitled3.png