This is really good as hypercasual games go. Won't say much about the graphics or sound since they're so plain, but the gameplay is nice. It took me a while to understand that the best strategy is to delay merging numbers and to keep a large pile of them somewhere so that you can add new numbers to a pile, or synthesize a new one if the next number you have to play is one that you don't already have on the board.
Having the game recognize when you reach 9 or attempt to reach 10 would be nice. Right now it feels more like an objective to reach 9, except that the only real effect it has on the gameplay is making it harder (a savvy player would postpone reaching higher numbers for as long as possible.)
I would also like if you could see maybe 2-3 numbers into the future rather than only the one you're about to place. I got a few game overs having to make 50/50 decisions, only to find out that I could have kept going if I had placed a number in the other vacant square.
Last, you could tweak the difficulty of this game by adjusting both the maximum number as well as the size of the board. The difficulty seems appropriate at 4x4 with a max of 9, but you could increase the difficulty by increasing the max number or decreasing the board size, or decrease the difficulty by doing the inverse.
Surprisingly fun game.