First thing that stood out was the viewport size. The game felt too small in the window, and the text was difficult to read. Our game is also 640x360, but Itch has a setting for the embed viewport size, and we set ours to 1280x720, which makes it better fit a common 1080p monitor. I did see the fullscreen option was enabled and I did try it, but while taking notes it is not as nice.
This is a decent start. The movement was pretty good, though I wish there was a reverse. I like being able to shoot enemies while backing away. Though that could make collecting the scrap harder.
The waves being time based, rather than based on the number of enemies felt off, and resulted in lots of lost scrap. My biggest problem was losing all the scrap from the last enemies I just destroyed. It reduced my options for combat, as I needed to stay close to the enemies to collect the scrap before time ran out, instead of being able to circle back and collect later once the threats were dealt with. With a set number of enemies you could even do an auto-collect of all the scrap remaining at wave end, with the assumption the player would go collect it while it was safe. While it is a game and you can do what you want, it did feel odd having everything reset after a timer ran out. Where did all the enemies, weapons fire, and scrap go? I like story, so maybe if there was a bit of story as to the setting with something about you warping into and out of the combat zone, it would feel better. I'm also a bit of a completionist, and wanted to keep deal with all the remaining enemies on screen.
The controls could use improving. I prefer WASD for movement in general, but especially if you are going to use the mouse for menus in the middle of the game. It helps if the controls in a game are consistent. If keyboard is meant to be the main input, then you should make everything work from keyboard. Either using arrows/tab to move between elements, or shortcuts to select options (such as using the number keys). On the other hand, if wanting to use the mouse for some elements, the keyboard should compliment it. Let one hand always stay on the mouse, the other do everything on the keyboard. In this case you could easily use WASD for movement, along with X for firing. Personally, I'd use WASD for movement plus Space or left mouse click to fire. Having the player need to keep moving their hand back and forth between mouse and keyboard, especially with that hand being the controls needed to move in combat (at least if right handed), is annoying. The combat controls worked fine when in combat, it was just the switch at wave start/end that was the issue.
The upgrades were good, and gave some nice options for how you want to play. I would like to see something indicating which upgrades replace parts, vs adding a new one. In some games you can have multiple weapons, so when I got my first new weapon I wasn't sure if would replace, or add an additional one. After getting it, it showed up again. Since it had a level, I got it a second time expecting it to level up, but it looked like nothing happened. Later I got a weapon upgrade option, and see that your upgrade system upgrades each of the 4 core slots, regardless of the item in the slot. In this case, it would be nice if the weapon you currently have equipped did not show up as an option to purchase, since it would not do anything but use up your scrap.
I was surprised the first time I got an out of bounds message. Space is big and I didn't expect an area edge limit. The message was clear, and the time to get back in bounds was enough that I did not have any trouble doing so. Once I knew about it I was able to plan around it.
Overall, it was a good jam submission.