Some of those suggestions like acquiring different guns and actual levels were things I thought I should do but didn't have time to implement since I challenged myself to make this in two hours including the art. I was also unhappy with the fixed shooting cooldown as a mechanic but some other things I tried like automatically reloading every frame felt too easy (lead to spam clicking being too strong).
I really like your suggestions of powerups and harmful targets, I didn't even consider those things. If I ever add to the game these are definitely going in.
Thanks for playing!