i comepletely agree with all of this!
i would also like to add that the upgrades arent really upgrades! even at level 3, the first monster takes 4 shots, same as level 1. so what is the use? a good way to upgrade is the first monsters takes 3 instead of 4 shots at level 2, then 2 shots at level 4.
I like where this is going and if you want more such things, let me know, i am glad to help 'for free' with ideas and such to make the progression feel better.