I just played your game, it is extremely fun but I think there is a lot of missed potential and to be honest I would never buy it in its current state. It feels like a tech demo rather than a full game. My unsolicited advice is as follows:
The art is boring. Most bullet hell games either have a mecha or anime art style with a lot of flashy effects. The closest in style to your game is probably geometry wars but your game lacks all of the color, warping, and particle effects that makes geometry wars look cool. You should invest money in the art.
It is very interesting that whenever you die in the game you are essentially killing yourself, or some sort of past version of yourself is killing a future version of you. There should be some sort of narrative element to highlight and expand upon this idea. I think it is way less interesting to sell it as "your enemies learn from you" when you could be selling as "a battle against all your past selves", way more emotional content in the latter, you're fighting all of your mistakes, all of your hubris, all of your near-sightedness, and every shot you take is another nail in your coffin.
To expound on this emotional element your character should be some sort of actual character, not just a green triangle... This doesn't mean you need a big backstory or cutscenes or anything major like that, something as simple as a drawing of a cute girl with a couple lines of dialogue will suffice. the thing you actually control can still be a spaceship but the player should know there is a character inside of it and they should know what they look like, you could make it so off to the side of the screen the characters face could pop up and react to what is going on. Go to tumblr and ask some type of fan artist to draw you a 'time traveling spaceship pilot that battles her past self and is really cute" and pay them for their work. Then when you die you can see that same character pop out of the space ship that killed you and celebrate ;)
You should rethink the gameplay progression and how the levels work. It doesn't seem clear to me why you split up the gameplay into discrete levels rather than having continuous play with 'respawning' enemies. I don't see why I have go through the weapon type progression linearly each time either (shouldn't I be able to collect or switch weapons or something?), reminds me of my main issue with geometry wars which was that I had to play for about 15 minutes before the game really became fun and that I had to redo that boring 15 minutes every time I died. Just start me with the fun gameplay or make the ramp up period very short.