This is the puzzle game I've played before, they're called 'ABC puzzles':- http://www.thepuzzleclub.com/abc-logic-puzzle.php
I realised that making the hints on the side refer to the middle letter / shape was better, as you could get away with not having hints on both sides of the grid (saving on screen real estate). Beyond that its a very similar game.
Amazing! Thanks for taking the time to respond in detail :)
I can absolutely see the 5x5 and hints being perfectly fitted to the playdate screen, similar to how Sketch, Share, Solve can't be more than 15x20 without requiring some UI/scrolling.
I'm really glad I know the name of the puzzle now, scratching the sudoku/minesweeper itch at the same time is such a trip!