I made a tower defense game for this jam as well, although I went with a slightly different mechanic.
I like the design choice of placement determining upgrade. I encountered the same issue someone else mentioned, which was the shots missing. I had this problem in my game too. I mostly fixed it (I don't think it's entirely fixed) by leading the shots depending on the dot product between the forward value of the weapon and the direction of travel of the target (the closer they are to 1, the more the shot needed lead).
Great job on the game!