Quite a solid take on a PVZ-style tower defense game!
I think the focus on upgrades is pretty neat! plus I like that the harder versions of the levels have actual new challenges instead of just spawning more enemies than usual.
Two suggestions sounds wise:
-The siren sound effect is a hell of a lot louder than everything else. I get that it's supposed to stand out among all the other sound effects, but perhaps you could tone it down a bit.
-Perhaps the Processor and the Refinery trashies could have a special sound effect for when they give you scrap, like a toaster ding or something. Serves as some kind of reminder to refill them again.
Also, there's a bug in the collection screen where if your mouse cursor is moving even just a tiny bit, clicking on a trashie/exterminator won't cause their info to show up. Nothing game breaking, but a little annoying nonetheless.