Pretty cool game! Overall felt like the game had a nice, addictive, 'simple to learn, difficult to master' vibe to it: felt great to blast away at ships, use the shields to block red bullets, and decide between moving side-to-side or teleporting depending on the situation. Great variety of enemies and upgrades, cool boss fights, great juicy graphics, intuitive and helpful symbols/colors and guides like the position highlighter, lively animations for both in-game and the menus, and gotta love that charming old-school Flash aesthetic as well: well-done all around!
If I were to offer any feedback for improvement, it would be thus:
Menus were a bit awkward and buggy in some cases. Overall I'd love if the buttons would react a bit more dramatically on hover, like becoming bigger and another color: right now all they do is sometimes turn a bit more grey, or not even react at all. Certain interactions also bug out, like if you select 'start game', but then use one of the menu shortcuts before picking a game mode, the two game options will remain on screen, whereas if you use the arrows, it will make the two game options disappear.
I found it a bit frustrating that while some of the item descriptions tell you what they do, such as increasing fire rate but decreasing damage, other item descriptions will just offer some sort of cryptic quote. Yes, it can be somewhat amusing and add to the mystique, but I'd prefer if you'd just go all in one way or the other: either have them all be cryptic or all be informative (with me leaning towards informative).
The shop option, while very neat and unique with the way it is done in-game by shooting at the various purchase choices, can be a little awkward and had me sometimes just wanting a more classic menu: I'd have times where I would overshoot money at an option and then have to wait for it to come back before I could purchase something else, and other times the continue option would take so much unnecessary ammo to activate. I do really like the implementation in theory, though, as its very charming, so I could live with it if it remains as-is.
I found it a bit strange that, despite selecting 'story' mode, there was no story. Not sure if it's not present for the demo for some reason, which is a shame as that would be the primary hook for me.
For the most part, everything is very clear and visible with important aspects being color-coded, but as the gameplay went on and I started to be able to fire more bullets and flood the screen, it became more difficult to track things, primarily being due to my bullets being a similar shape and color to basic enemy planes: maybe some more differentiation would be nice, like enemies always have a certain colored outline.
There's also a lot of weirdness that came due to rebindable controls. Don't get me wrong, it's nice to have rebindable controls, but when you're playing the game, the in-game tutorial will still be based on the default controls (so for example, it will still say 'A' and 'D' at the beginning despite me switching it to arrows keys). Furthermore, while the tutorial menu uses the rebindable keys, it uses weird language like 'Mouse0' and 'Mouse1' instead of Left Click and Right Click, or those nice mouse symbols that are used in the in-game tutorial. Basically I wish the tutorial would be as nice looking as the in-game one, but respect the rebinded controls like the menu one.
Finally, as much as I do like the game and have fun with it, I'm just not sure whether it feels like something I would actually spend money on. Arcade/score-attack games like this always feel difficult to spend big money on since they can feel so short-lived due to a lack of long-term progression or story hook, and not to mention that there are so many other games of this ilk vying for competition, with many of them even being free. Best of luck, though!