Basically I wanted to include everyone who donated $X+ to the game in the credits.

Who donated to your game is private information. Unless you have consent from those users, you can't just use their user-names and display them somewhere.

If you want to dish out rewards that are files, that is easy. Just put up a file that has an indivual price. Only people that paid the required amount will have access to those bonus files.

If this is about an unreleased game, https://itch.io/docs/creators/exclusive-content might be used for this. I do not know, if it works for previous customers or only new ones. Since the customers have to claim this, you do have consent. You can also ask for a information, like, how the buyer wants to be credited in your game.