Glass cook top cleaner/completely coat the inside with oven cleaner after running a self clean. That should come out easy. Sending a letter to the previous renter calling them a filthy pig (using industry terms, obviously) is going to be a joy
They make some great products that should be able to clean it. It includes a razor that scraps the dried burned on stuff. The right side looks really bad so idk.
Try cleaning- don’t send final bill yet. If you can’t get clean, then charge for stove top replacement. I think charging for the entire appliance is extreme.