Well, I wasn't planning on playing as long as I did, but that's a good sign.  Got up to like 20 kills.  There's a lot to work on, and a lot of jank you're probably aware of, but I feel like the simplicity works, and is kind of compelling.  Here's some impressions and notes:

1.  I used mouse and keyboard, so maybe not ideal, but I felt the mouse aiming was difficult.  Also I realize I should have played full screen, because my mouse kept going outside of the box.  

2.  I never was sure which way I was going to fire, right or left, since the ship keeps turning.  Maybe some sort of visual indicator to help you keep track (like red on one side, yellow on the other?)

3.  Some of the upgrade buttons didn't work for me (Health and Cannons), and they don't even look like buttons (so I didn't know I could press them at first).  Also the other buttons (buy, sell, repair) are overlapping each other fo r me.  I'm on Windows, Edge, and not full screen

4.  Very frustrating how slow the ship is at first, but feels much better as you upgrade  the speed (and once I figured out how to upgrade).  So this probably is a good thing, but just make it clear to players that they can upgrade speed.

5.  I didn't know the green circles were aim assists.  I was trying to hit other ships with them at first :).  Maybe making them like a gun sight thing or something would be good?

Anyway, I know it sounds negative, but I definitely got into the game, and there's a lot to like, so this is a good prototype, and I think a lot of the above can be fixed pretty easily.  Then if you want you can start adding things like more enemy types, different weapon upgrades, different types of islands etc.