Where to go after "olympiad math"?

I don't think there's a lot of sense in learning things that will be in your high school classes. You might want to take in some elementary set theory. Once you understand that, you can start plunging into any of the modern formal math subjects -- axiomatic linear algebra, group theory, graph theory, and so on. You don't have to dive too deeply into any of them.

One thing that really excited me when I was your age was reading through and making sense of the works of Raymond Smullyan. He writes logic books with a very approachable framing story that eventually leads to very deep and influential topics in logic and computability. There tends to be a lot of overlap in his books, particularly the first third, but I think any of his logic books are worth reading. My personal favorites are To Mock a Mockingbird and Satan, Cantor, and Infinity. I also like Forever Undecided, although that is particularly challenging.

Also, if you like coding, there are avenues for developing your catalog of languages and skills with algorithms and dynamic programming. Things like the UVa Online Judge can be fantastic things to dive into.