Saw an interesting discussion about ATLA that may or may not be useful for this project:

"In my opinion, I feel like the a lot of the early episodes could benefit from being downscaled, and then reupscaled again. These blurays are pretty much what the DVDs should have looked like. If only they could have given it a true remaster.

I was actually tinkering with this show and ESRGAN earlier today. Much more demanding than waifu2x of course, but my goodness, the results after downscaling, dehaloing, and upscaling again were pretty impressive. Here’s a frame I had tried this on."