I know it's just a prototype and probably gonna tell you something that you already know, but maybe you'll find it useful.

I think it's neat that you have to hold the button down to move the reticule, BUT maybe it'd be better to use a mouse aiming. Make it like a twin-stick shooter but in 3D. 

You definitely have to add some indication that you're shooting. Flying bullets from your character with a super speed would help it a lot. 

I didn't like the color of the enemies' bullets.

Since it's unity and you can make it fully 3D I think it could use a small camera sway when going left/right so the plane is moving just a little. 

I've made a prototype myself of a similiar game, hit me up if you wanna compare our ideas!