I found the game to be a lot of fun. Very tight gameplay, very original progression system, very satisfying.
The one gripe I had was with firing the main weapon. The only potential downside I could see to firing the weapon was that you might hit an enemy offscreen and make collecting their cash more difficult, which isn't a very intuitive or fun thing. Essentially, you should just always be firing your weapon so you can hit the enemy as soon as possible to maximize your chances of winning.
I've two suggestions for this. First, add a Toggle Firing option, so that players can choose either press down to fire or press to start/stop firing.That alone would make things more comfortable.
Second, give a small incentive to not fire your weapon. There's several ways you could do that. Add a heat mechanic so that weapons overheat if you fire them too much, make firing use up some of the mana for spells. One idea that I had in mind is that one second after you stop firing, you go into a prep mode, which reduces the cooldown on your spells by a set amount. That way, you give the player a bit of a reward when they beat a wave quickly and have a breather before the next one, making them actually look forward to those moments, rather than dread them as "bahhh, nothing is happening now".
Either way, it's a rather minor issue, and I'm looking forward to the full version of this game very much.