Prioritize brokerage account for child instead of 529?

Executive Summary

My son will be 5 next month and I have $36k saved in his 529. Even if I don't contribute another dollar, it will be around $87k by the time he's 18 (7% growth). I want to create another bucket for him in a brokerage account for added flexibility so we can help him with a house down payment, car, wed...