This is really beautiful and surprisingly addictive. I think you also did a good job with the limitation! You have a really fun gameplay loop, if you just added more content I think it would be even more fun! Still, really good I very much enjoyed this. I also wish there was an end condition. Maybe you could add a gamemode where you have limitied time and have to kill as many guys as possible, though I think you'd need more variety in tools before that happened. If not that, just a final upgrade worth like 2000g would be good enough to say "I completed this game".