My sole beef with this game is that I found myself retrying some levels dozens of times after having mentally solved them, simply because they required unusually precise timing / controls for a puzzle game.
I played this game three years ago and loved it, 5 stars. Then the badge of the week came up, and I'm going to beat it again, because 5 stars aren't enough.
This simple little game is simply brilliant!
I really appreciate your stated design goals, but the difficulty is simply non-existent. Using the most obvious tank designs, I feel like I broke the game, in the sense that I was murdering everything and taking almost no damage. It turned into a game of patience, simply moving in one direction until I met the boss and hit the max level.
Perhaps the difficulty as you wander farther out should grow exponentially. Maybe also watch the number of gun points allocated per level, as that is what I exploited to put lots of big guns on a tiny bubble, making myself very hard to hit yet spraying bullets everywhere.
I like the idea, not fond of the execution. There is no indicator of how many enemies remain in a level, and some of them seem to go on forever... For some reason, my ship kept getting stuck every 15-20 seconds. I still had the key pressed but it would simply stop moving until I let go of all keys and started moving again.