>>24516166kant's main presupposition is that a priori synthetic knowledge exist, which he supposes because of his philosophy of mathematics. his other main presuppositions are just the particulars of his analysis of consciousness through the relation between logic and the pure understanding which he uses to "derive" his categories, which particulars aren't necessarily relevant to his main insight about consciousness which is that it requires some kind of conditions to exist.
all the stuff about "noumena" "phenomena" and "things in itself" is really not that significant a part of Kant's philosophy, it's overblown and misunderstood by people who haven't read Kant but need some kind of simple idea which they can use to categorize him in their minds. the distinction between noumena and phenomena is something that kant made specifically to refute the notion that humans have perfect knowledge of the thing in itself because their intuitions of it are "intellectual" which in this case means that the intuition itself creates the object.