I tried my best with the shading and I'm not really happy with
You don't need to update a pre-scaled version of the art. PJ has a zoom feature, so uploading the 1x size is good. When you must scale your art, only do it in clean multiples (2x, 3x). Your preview image is scaled unevenly and looks terrible as a result. The preview images are subject to the same rules about pixel art as the detail files. If your art doesn't fit into the maximum size even at 1x, crop it instead of resizing.