Fun basic gameloop!
Could use more game juice, particularly I would like to feel awesome every time I hit a helicopter.
Also, whenever a city gets hit, it should feel worse (example that comes to mind is when you take a hit in "Hollow Knight").
The upgrades could use more visual variety. It would be nice to also have something that prevents me from accidentally buying an upgrade because I was clicking to shoot, like maybe a 1 second delay before you can buy an upgrade.