I expect that there are plenty of courses for them to take in college. My older two each had at least one kid in their HS class who had exhausted all of the HS math offerings by end of 8th grade (so they completed multivariable and linear algebra).
We are fortunate to be in an area with a lot of four-year colleges; there are six four-year colleges within three miles of our house. These kids attended math courses at a well known college within a mile of the HS. I know that the one kid went to MIT and started taking graduate level math courses freshman year.
Engineering really doesn't involve that many mandatory math courses, or it didn't when I went to college. Three semesters of calculus, linear algebra and differential equations - that was all I took. If you already took these courses at a college, you might not need to retake.
My middle kid, who took linear algebra senior year of HS, got a degree in applied math (as well as a humanities degree). There was plenty of math to take, and she took linear algebra again in college, except the college course was far more proof heavy.
