The rate of fire seemed very low, I was expecting to be able to fire again as soon as 1 shot hit.
I don't agree that the game needs a lot more complexity. You have 4 different fire buttons, and you're shooting from a moving position, that's pretty much enough complexity from a control standpoint.
I did try repeatedly to go to track 2 before I read in the description that there was only one level so far.
I think the game is currently pretty good, but a bit too short.
Larger tracks would obviously add to the game, and likely would be additional levels.
I would suggest making each level slightly bigger than the last.
I can see adding in some things like road blocks, or bounce points.
Try not to increase the difficulty too fast as you make new levels.
Remember that the creators understand the controls, and likely have practiced a lot.
New players will be having to learn all aspects of the game from scratch.
Some things that might make the game more challenging:
Targets that must be hit from a specific direction.
Targets that deflect a shot without stopping it.
Targets that rotate when hit from the right direction.
Rotating targets would also likely have another property, like one of the above.
Imagine if the 2nd track was exactly like the first, but the crystal had to be hit from all 4 directions to advance...
Or if the crystal had to be hit from the left or right, but the last rocks could only be hit from top or bottom, requiring them to be cleared before the crystal could be shot from the side.
A figure 8 track with 2 crystals could also be done, and would work like the first track doubled.