This is actually rather well made, everything is smooth, the menus work well, the graphics are decent for the style, there's good separation between the elements (like bullets and such) May I ask what the base resolution is? Also - what are you using to make the hit enemies flash white?
For me controlling the action with the left thumb was really awkward - not sure if a horizontal shooter really works that well on a mobile/tablet. Placing the phone on a table and using an index finger worked better.
Also, if I could leave some suggestions - the player craft and enemies feel a little lifeless - perhaps a little animation (or, say, floaty up/down sine movement or mild rotation on movement) could help with that. Also since the performance is so good you could probably add some particles to weapon impacts/explosions and such - might look neat.
Other than that - well done, hopefully it does well for you on the market.