I'll start by saying the visuals in this are quite good, I like the spritework of the enemies, the way the player's bullet patterns move, the general choice of color palette here, there's a lot to like on that front! Very charming stuff.
I like the core of the gameplay here being focused on power ups and going for as high of a score as possible. Has a rather nice "just one more try" feel, which is very gameboy era video gaming. And like, I do especially like how those power ups work. Again gonna mention how satisfying the way the player's bullets move with certain upgrades gets.
And I really like that every few seconds you get to choose one power-up and potentially have to risk a slightly inconvenient position to go for said power-ups. That's risk-reward! That's pretty good design!
And the reward for getting those in feeling significantly more powerful over time? That's good payoff. Oh yeah, also? I think the inability to aim up and down actually makes the gameplay better, since it means you have to be more careful about how you move about, and makes getting those power-ups that much trickier to do yet that much more satisfying when you do it. Surprisingly clever choice there.
I do, however, have my issues. I ultimately like this game, actually I really like it even, but it's a bit flawed, and I may as well get into what I mean a bit since hey, maybe some of this can get fixed after the jam. (The first issue or two I mention in particular feels like a rather good candidate to focus on fixing, if nothing else.)
The big one is that, as far as I can tell at least, there is a complete inability to reset the game after dying without rebooting the game entirely. This does detract from the potential of the gameplay loop, I feel, since it feels like a "just one more try" kind of game, yet makes it weirdly inconvenient to give it that one more try to get into that kind of flow of trying over and over again.
A very minor one is that having the game track your best score in some way would also be nice, to have that further encouragement to push for a better and better score.
Also, lastly, due to exactly how you've gone about using your 4 color palette, combined with the sheer number of objects on screen at once, this feels a lot less like gameboy and a lot more like gameboy color, so while the color count never technically is more than 4 on screen at once, it's worth a mention.
Anyway! Thanks a lot for reading this and, if you're the dev of the game reading this, for making this game! I'm glad I sat down and played this one. If I didn't have more things in the jam to be playing, I'd probably be trying to sit down with this and play it more, actually... I feel like that's a good sign you're actually really onto something here.
Heck, to that end, genuinely, consider making a larger version of this some time if you ever find a chance. Not like, significantly so, maybe just something like, adding in a soundtrack and such, just generally polishing up and cleaning up what you have here more. I know I'd play the heck out of it myself. That's ultimately up to you though, of course, I don't blame you if you'd rather move onto other endeavors!