There's a lot I really, really like about this game. The arm mechanic's super neat, the self-imposed risk/reward is cool, the gamefeel's pretty damn impressive for a jam. I like that it's reactive, but also puzzley. You nailed the addictive, "just one more try" loop of a Binding of Isaac-style roguelite and/or a masocore game. (To the point where this comment took a long time to write, because I kept reopening the game to try again!)
I get that you're going for a very twitch gameplay style—and I really like the twitch gameplay, even if, like other have people said, I think player speed's just slightly too fast (though maybe it could be fixed by adjusting the controls?)—but the enemy bullet speed feels a bit unfair. I spent a lot of time going "where did that come from" instead of "oh yeah, I should've seen that." (Oftentimes my instinctive attempts to dodge bullets ended in me running straight into the bullet instead.) Maybe this is just because I like bullet hell games, but I feel like you could get similar panic without the player feeling cheated if the bullets were slower and had smaller hitboxes, but fired more often? Dodging leftover bullets could give the player something to do while waiting for the hole to open, and could also make the spikes a bigger part of the challenge. Just one potential adjustment.
Similarly, I really like the idea of conserving time by firing shots that kill multiple enemies, but I feel like opportunities are slim, and risk tends to outweigh reward to the point where I don't do it as much as I'd like to, for fear of running out of time. (I love the "limited time for a set number of levels, with rest/reset areas" mechanic, though. Great idea.) Maybe it could be interesting to lean into the "combo" idea and add a bit of ricochet to the player bullet, or program the enemy AI to cluster more.
All that being said, I would play the hell out of a full-length version of this. It's so easy to imagine potential obstacles, enemies, modes with different time limits, what ways you could spin and tweak the mechanics, what a boss in this game might look like...it really has a lot going for it, and that's true even in its current form. Good work!