This is really helpful.
Do you know if there is a guide or something to do the USB backups though? I know it's a simple thing, but I never actually learned how.
I'm sure plenty of guides have been made, yes. I wouldn't know where to link you though.
But it's fairly simple. All there is to know is that if a USB is connected, the Wii U will prioritize it when saving something, and so the file on the Wii U will act as a backup. Once you've installed the USB and copied your Splatoon file on it, simply proceed like so:
-Roll a bunch of gear as much as you want/can.
-Now your USB has the file where you've spent some/all of your money, and the Wii U has the file where you haven't.
-If you didn't get anything you wanted, you want to take your money back, so you need to copy the file from the Wii U to the USB.
-If you rolled something you want to keep, then you need to update your backup, so copy the file from the USB to the Wii U.
I hope this clarifies things a bit.