Pretty cool vibe!
I really feel like I should be able to move while shooting. The gun feels cumbersome and restrictive as it stands. It seems like you could allow movement, and then compensate by increasing the enemy fire rate or speed or something. The whole thing might feel a lot more fluid and intense as you weave around enemy bullets while managing your heat level.
Apart from this it was really fun! Great work! :) It's awesome to see how low-fi you can get Unity games to look.