It’s crazy how deep this game is.
Check the "unlocks" section on the main menu to see all the ways to get more characters and upgrades. I've actually been focusing on all of those unlocks now, as there are so many specific weapons I've never fully upgraded or matched together with it's partner weapon, and so many characters I need to get to a speciifc level or time limit with. I just noticed one of the super pricey upgrades I can do now is a permanent minor stat upgrade to a character I theoretically can do each game if I have enough gold.