As a big Picross fan, this is right up my alley! I really appreciate your hard work
That said, you absolutely need to check your puzzles to make sure they're possible without guessing. Just based off of the screenshots, 3/4 of the puzzles (Kirby, Link, and Birdo) seem to be impossible. For a puzzle game, this is a gameplay mechanic failure at the most basic level. Plopping pixel art into a grid isn't going to cut it most of the time.
Still, I wish you the best with this project, and if you ever do an update that fixes these issues I will be all over this.