After multi there isn’t necessarily one clear path, but lots of options. My son took multi/linear algebra (at his HS) in 11th. Then in 12th he did a number of independent study classes - one was real analysis, one was algebraic geometry. He spent 2 summers at a number theory camp so didn’t need another class. He had taken a 1 term diff eq class, but will retake it in college since it wasn’t a full semester class. Some classes may not transfer, but that will be school specific. In the spring have to work on this with kid 2 - he will finish multi in the spring and have the math department at the university open to him. Probably he’ll start with linear algebra, but not sure what else he’ll take.