Itch utilizes support for patreon, so you can give rewards to patreon subscribers. You could implement most of the functionality you describe with it. Have you looked at that?
Also for paid updates, you can set a “different price” to an updated version, which if I’m not wrong would force the user to pay that price to have access to that file. However I’m not sure how users will like that, but that’s an entirely different topic.