The simplest thing you can do to make this enormously better is have two buttons that let the player turn left and right. It doesn't feel good to overshoot. And if giving the player more control makes the game much easier, then compensate by having more enemies, and it would be more fun that way.
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This is a feature! You can only turn in one direction or stop, not go back. It should make it a bit more of a challange.
I planned to add more guns (and a flaming sword on one side) to the other sides with updates, to make it easier, while enemies come in faster and stronger. But did not get this far.