I've found a good basis in abstract math makes a really wide range of mathy stuff accessible. The first two chunks I would tackle from a high school math + coll

I've found a good basis in abstract math makes a really wide range of mathy stuff accessible. The first two chunks I would tackle from a high school math + college calculus foundation would be set theory and group theory, and though it's an oldie, I'd honestly recommend Russel and Whitehead's "Principles of Mathematics" for the former as a start.