Thank you for the mobile ideia, I didn't thought about that. The 'gain speed' is a good idea too!
About the shooting rate, I tried to let it slow in the begging and faster with the upgrades, but the hit indicator I should had do some animation or something.
About the engine, I used Unity a lot of years ago and the only thing I remember is that I didn't liked. ^^' I'm using Godot for only three months, but I found it to be very intuitive.