It may be for a Sunday. Grabbing a taxi might even be a better option than the bus if you want to get there very early. And getting there VERY early sounds like a great idea. I did a morning hike in May (not sure of the time, but arrived in Nocelle for lunch) and there were already plenty of people and it got hot. Don’t underestimate the sun. You’re exposed for the majority of the hike.
I’d check the schedule at the other end too, from Nocelle to Positano. The problem with these infrequent busses can also be that there’s a ton of people trying to get on them. A taxi down from Nocelle might be part of the plan too. You can walk down from Nocelle, but it is a hike on its own.
I’m pretty anti-PotG, but understand if your heart is set on it. If you’re not sure it’s worth the effort, I’ll tell you it’s not really. As an alternative, a day in Ravello offers very similar views. Or in Positano, take a bus or taxi to the top of town and hike down.