The games best core strenght is that addictive gameplay. The core mechanics work great, picking the game up and playing it right away feels natural and the game creates a sense of flow immediately.
The game also does a good job with the nice pixel art look and feel.
You could have been more effective explaining the controls in the game. I just found out there is a settings menu by re-reading the page here, and X is not the most intuitive key to press to start a game anyway.
You should also focus more on giving the player a sense of progress. Passing by planets or some nice background art or giving the player constant XP for unlocking new ships might be a good addition to the game.
In retrospect it would've probably been a good idea to show the controls on the menu, if at least for the first time playing. I actually might patch this in some time.
The reason you press x to start is mostly due to it being the button you press the most often, and so that you can see how shooting works, as well as how it helps you manoevre.
I did try putting other stuff such as planets in the background, but felt it was a bit distracting, especially as not paying attention for just a few seconds can cause you to lose. I also wanted to make the setting feel more barren/abandonded.
I also don't really like implementing XP/levelling systems just to create the feeling of progress - the only reason you can change ship at all is to make what is basically an achievement system feel more rewarding. Maybe that isn't something I should disregard outright, but I feel it would only encourage one type of gameplay, whereas the different tasks require you to focus on different aspects of the game.
Some of the areas you praised (flow, pixel art, game feel), however, are things I have been trying to focus on and improve in my games and I'm glad you appreciate them!