“we found lots of highly successful people who had “that one class” or more that they struggled with.”
This is a very good point. For any student, no matter how strong, you just might run into a tough course with a bad teacher. For me this was Control Theory as an undergrad. For the next student it might be something else. There is also the risk of a course that you just don’t like at all.
“The point isn’t to add extra courses after the semester starts. My son always registers for 18 credits.”
I think that this is a good plan. I used to do the same thing. I always dropped a course by the end of the second week, usually by the end of the first week, leaving me with a regular course load.
Getting a bit ahead is also a good plan so that you can drop something if needed. One daughter is about to take a summer class that will leave her well ahead of schedule. We have made sure that she is very aware that this gives her the option of dropping a class if needed in the fall semester (which is going to be a heavy semester for her).