This is really good, would wishlist in a second.
The progression feels nice, although it might actually be a bit too quick. I feel like I can barely see the effects of my upgrades before I can buy 5 new ones.
One small issue - I don't like how the popups in the skill tree are at absolute positions and can go outside of the screen. So I have to scroll back, so that not only the node, but also the entire box is inside again. It would be better if the game moved the popup to the edge of the screen instead of cutting it off.