This is almost looking like a polished game, it's amazing what good assets can do for a game. If you made it so it's hold-to-rapidly-shoot, I think it would make it 10x more fun. Also, I would switch the movement controls from arrow keys to WASD as it's more comfortable.
I know this is just a demo you made but I think it's a cool concept and those might be some easy things to add to make it feel more fun/smooth to move and shoot.