Answering the object-level problem "my grandma wanted to play some sudokus on her computer", I really enjoyed the Cracking the Cryptic-affiliated game "Classic Sudoku", which is available on Steam, although some of the puzzles are really hard. The puzzles are all handmade, and many of them have some specific reason to exist: e.g. there's at least one which is clearly intended to teach you the swordfish pattern, and there are a few which appear to be built around some beautiful one-off ideas.