Minor error -- you used the "Skullface Lord" stat tile for the Guardian Golems but didn't rewrite the data on the tile (page 17).
I'd recommend not upscaling the images, the information isn't there afterwards, you can induce moire in textured backgrounds and the temptation is to sharpen and overprocess the upscaled graphics, destroying the artist's "line". Been there, tried that, gave up and went with the original resolutions for my own series. 1600px is well good enough.
By the way, thanks for doing this series. I like the art and the story.