I like to believe one can't be 100% sure of anything, for one can't be 100% sure their brain functions correctly (because if it doesn't, it may mistake itself as functioning correctly as a result). Is there a name for and/or existing material on this theory?
I am going to disagree with the "radical skepticism" label. "Not 100%" is very far from "most likely impossible". That certainty is never justified can also mean that it is 99% justified 99% of the time, and I would call that rather overconfident. Depending on specifics OP stance can describe a variety of positions including Quine's empirical holism for example: any claim is subject to future revision in the face of recalcitrant experience, there are no a priori certainties. About 50% of contemporary analytic philosophers subscribe to naturalized epistemology, and they are hardly radical skeptics.
Radical skepticism is the term you are looking for. Descartes Meditation is an early work that starts from this position.
Pyrrho got there long before Descartes, and stayed in the vein for a longer time before betraying its basic principles. Sextus Empiricus' rearrangement of Pyrrho is the closest thing we still have.
Descartes only wishes he could manage to be a radical skeptic. I don't think the natural exit point from true radical skepticism involves three separate proofs of God, and all the other trappings of Descartes.
And as for all the disparaging comments. Bear in mind that major world religions, including branches of Hinduism and Buddhism think it is important to be radically skeptical, at least on a theoretical level.