Finding a balance is hard & tricky. Once I found out that my son had such low processing speed and working memory I realized for HIM we really needed to cut back. Other kids who whip through this kind of work (such as outlining the book) can handle more classes.
We have one IB high school and the others offer AP. I remember, at one point, researching IB classes and found there is some controversy about whether or not it's just "more" instead of "better" work. Another thing that is tricky is the students needing intellectual peers. I think my son is less "bored" in classes where there are a lot of other advanced learners, but it's really a social thing as much as an academic one.
It seems to me you've put a lot of thought into the schedule selection--I hope you will keep us updated!
