Thank you so much for playing and im glad you enjoy the game. That is a really good idea for sure, too bad this was a small Game Jam game i made a long time ago and i haven't continued it ever since.
About towers above advanced not registering it may very well be a bug, i honestly don't remember, or maybe it is a bug with the way the game calculates your gold and thinks you don't have enough to buy the new towers. I honestly don't remember, but im pretty sure i used to be able to buy the better towers.
Im sorry i never fixed those bugs, the game was intended as a small Game Jam game.