The noise gets quite grating. Maybe have a little randomness in pitch and length, and. Have a milder noise when doing unimpressive things, and a slightly longer and more jubilant noise/animation when doing more impressive things.
Whether a move will make the puzzle unsolveable feels very random, and it's very punishing for how random it feels. Maybe instead of resetting, you add multiple new numbers? So it's a setback but not a setback to the start of the level.
Zen mode: disable hints. Track something other than time, e.g. they should try to combine numbers as few times as possible? Maybe like. They have a counter that starts at 1. When they combine 2 exactly equal numbers, if those numbers aren't prime, the counter is multiplied by that number. You want the number on the counter to be as high as possible.