So I am usually in the “cutting it close” family, but haven’t had a wicked bad experience yet. I would say you would be fine to be walking out at 3:30 as long as you are relatively savvy. Like others have said there are like 3 ways to get to the Poly, ferry to TTC and walk would probably be my choice, but you can monorail or take the launch. So as you walk out the exit, eyeball which seems to be the next departing option. None actually take that long, it’s waiting for the next one that is the variable.
That being said, if you don’t feel like you can make the game-day decision on transport or are wrangling a large family/young kids, you might want to leave earlier. I just get the impression you were looking for “last possible time to leave.” I don’t think the post-midday-parade exodus is as bad as the evening parade one.
Also in the “cutting it close” camp, if you want to eat around 5 and you’re a little late to your 4pm reservation, you will probably eat closer to 5. I guess it depends if you are ok grabbing a drink or waiting at the Poly. Ohana is honestly ALWAYS backed up in my opinion whether you arrive at 3:50 or 4:10 you’ll probably have to wait.