I am so into this! I love a one button game, worth considering making a touchscreen compatible build.
Controls work well, and the graphics make it nice and clear where you are and where you're going to go. It might be nice to visualise the momentum somehow (an engine trail maybe?).
I feel like the level design is at it's best when the ship is moving fast. Perhaps you could introduce some mechanics to encourage this (starting momentum, homing enemies, time limits etc.?)