I want to go back as far as necessary in order to relearn and solidify my understanding of foundational math concepts. I did not enjoy high school math and only completed the sequence up to Algebra II as a Sophomore before really taking an interest in the subject after graduation. I studied computer science in college and took as many math classes there as I had time for, foolishly skipping remedial pre-calculus in my desire to learn more advanced concepts. Despite my shaky foundation, I was able to complete the basic calculus sequence in addition to discrete math and linear algebra, which I enjoyed immensely. While I was always able to grasp the concepts, I became increasingly aware of my lack of foundational knowledge and experience, as I struggled with long computations and proofs.
I am now self-studying linear algebra in greater depth and really enjoying it, and I realized that I could address my limitations by relearning high school and maybe even middle school math. I want to intuit the foundations so well that using them is as easy as riding a bike and they give me greater insight into more advanced problems. I know that's a tall order, but I consider this to be a personal project that will take multiple years to complete.
Also, I have a friend who participated in the high school math Olympiads and studied The Art of Problem Solving as a student. Would drilling competition math as an adult be a good way to deeply intuit the foundations or waste of time?