Very fun game! Fast and frantic. Game feels really good and satisfying. Some minor feedback (I know it's a jam game, feel free to ignore).
-When I first started playing (before I got used to the timing) I would often shoot the upgrades by accident before I knew what they were, since I just kept shooting as the level ended.
-Enemies warping in right on top of you can be frustrating, especially the ones that shoot bullets on death.
-Generally when a lot of stuff is warping in visibility is poor. Also, player's bullets stand out more than the enemy ones, and it can be hard to quickly orient yourself after dying when a lot of stuff is on the field.
-I appreciate the minimalism, but it would be nice if more information was on-screen. I often forget how many lives I have and can only see it when hovering over the upgrade. And with any upgrade that affects stats it would be nice to display the current value and how it will change.
-This is really minor, but it's not clear to me how the waves scale up and how that interacts with the # enemies and # enemies required to clear upgrades.