If you could deplete the 529 on younger kids tuition then use cash on hand to reimburse oldest kid the delta, it would be gifting and ok. Doesn’t sound like the path you want to go. Maybe lower state taxes if he moves to lower/no tax state?
Your logic sounds good. Just like a 401k. Deferred taxes. Pay now or later. I am not an accountant but have dealt with this issue. 2 sons. One full pay, the other nice scholarship.