I think given the fact you have to leave between 6:00 and 7:00, I would go with breakfast. Even if you have a 5:00 p.m. ADR, that doesn't mean you will be seated right at 5:00. Also, if you are driving, having a big meal like that might not be too good right before you set out! If you are flying, you definitely don't want to run the risk of being late and worried you will make your flight. I love dinner at Chef Mickey's, but I think for your specific needs, breakfast might be better. Now if you can get say a 4:00 ADR that might make a big difference, not sure what the earliest seating is at Chef Mickey's. Either way, what a fun way to end you WDW trip!