Wouldn't these be RPT with a known schedule.
So you're saying you're happy to lose a stack of time en-route on the off chance everyone departing from closer aerodromes gets away exactly on time?
You also need to get your head around the idea that times at fixes doesn't separate you from other traffic. All this you beaut automated stuff works wonderfully well when you're on your own but is ****e when there's other traffic in proximity.