Son’s large Wisconsin HS did not weight grades and UW uses only unweighted grades. Your HS stats look good for being admitted, worth your while to apply.
Students are admitted to the whole university at UW, regardless of the intended major. You will discover more about your self, likes, abilities and possible majors in college. Most students are either undecided or change majors once starting college. You will start with courses that will lead to a major or at least help fulfill breadth requirements. Coming from California you are not a foreign student- spending a year abroad won’t matter. Plus, you are a US citizen. It sounds like math/science is your forte- many good departments for that at UW-Madison. It does not hurt to use your senior year to learn the most in your weaker areas (although still strong) - writing and reading skills benefit everyone.
So- enjoy your senior year but do not become a slacker.