Great idea and execution! Just enough enemy types, guns feel almost perfect to use, and upgrades feel really good to buy and use. Earning money and most upgrades are well balanced for the most part (more on that later). Flavor is really good in chat and the upgrade menu has a lot of personality.
If you ever come back to this game:
I wish there was some incentive to actually read the chat, although it is a cool feature, I think leaving the chat itself (I understand the chat upgrades DO gameplay stuff, I'm referring to the chat text) as just flavor a bit of a missed opportunity, but I do understand it would probably break the game's flow too much.
As an AVID fan and high-ranked member of the "Bullet Count Upgrade Lovers Anonymous" community, I kindly ask for a complete rebalancing of the bullet count upgrade, because as it is it significantly worsens your run with each buy. I would prefer if the speed-related upgrades were more significantly pronounced and got significantly expensive after each upgrade as well, but that's just personal opinion.
Also, you might want to add a max fov to the minigun.