Okay, so, first thing's first!
I got my UpgradeManager class working successfully! Yippie! That'll make adding new upgrades a lot easier in the future, since now I can create them all with one object instead of copying and pasting the same class and methods over and over.
I also ironed out MOST of the bugs, hopefully. With this sort of program, it's less game-breaking stuff and more... logical errors. I keep stumbling across oopsies on MY part that lead to wrong numbers showing up in places. Nothing too major, just a little extra thinking on my part.
Here's how she's looking right now. I also moved around some of the things. Next on my to-do list is adding a dark mode, and maybe updating the interface a bit more. No shame in polishing what I have now as long as it works.
I've got it hosted on a private itch.io game at the moment, so I don't have to keep sending people suspicious-looking download links! That game is here: https://essiedev.itch.io/click-that-cat?secret=eK9ycrrOORSk34F2kuqmWR6xyg
Hopefully I'll be able to do a bit more with it tomorrow! But I'm quite proud of what I have so far.