Hey, thanks for feedback!
I am already working on the quickcast option, which would work exactly as you described it with holding the button.
I will also in the next update buff cannon towes and slightly nerf arrow towers, but the main thing will be next week when I finish working on armor system and 2 new towers. Cannon towers will be able to break enemy armor which will increase the physical damage received by those enemies and will have a stun upgrade. One of the new towers will be able to apply a debuff on enemies that will increase magical damage. On top of this I will add new upgrades some of which will help the Fire tower.