Great game but I would add some small UI changes:
1) make the upgrade menu take up less space, you have to open and close it constantly to get coins from the right side of the screen. (maybe turn the upgrades into small icons with the text explaining what they do appearing once someone hovers over it, or have the blank space in the pinned upgrades tab be smaller)
2) add more obvious explanations of the effects of skills. I couldn't find any explanation of what they do other than what the icons and titles implied.
3) if possible a sort by price option for the upgrades would be nice.