Nice job! I also did some pixel art experiments several weeks ago with grid-based constraints via Img2Img with Gemini 3.0 Pro that might be of some interest:

https://mordenstar.com/other/nb-sprites

FYI there are several nice tools for quantizing to a grid as well as reducing to a smaller discrete color palette that I recommend using as well (unfake [1] and pixel-snapper [2]).

[1] https://github.com/jenissimo/unfake.js

[2] https://github.com/Hugo-Dz/spritefusion-pixel-snapper