Thank you for the extra feedback! Visuals and sound are the main focus for the final version so that's coming.
Other people also specifically mentioned some of these things which I was oblivious to as the maker! "Level transitions" are explained in a different comment below, so I will look into adding some sort of notification on the screen.
Also for the upgrades you can't buy, I assume it was just not easy enough to see that the button was slightly grayer?
Hold to fire was also mentioned, but I still have to decide on it :)