The first shuttle starts at 0450 from Akandana Parking Lot. Yes, I advise you take the (for your) earliest possible shuttle. Not that later shuttles get congested, but it's just that you get nicer views in early morning hours for Kamikochi Highland. The later you come, the hazier it ususally becomes.
To reach here, you need to make a right at the post office when you drive through the downhill road and get to Hirayu (don't go straight here, if you do then you'll be in the middle of the ryokan-clustered area). Before that, you'll find Route158 is pretty straightforward when en route from Takayama to Hirayu, the latter of which is a small onsen town. But beware while on the curvey uphill road; on your left-hand side are rock walls; on your right upcoming traffic. That said, if in dawn time Route158 is almost (always) deserted.
Anyway, early-morning hrs are far nicer than late AM-ish hours, when bus loads of tourists begin to jam up the Kappa Bridge area. Believe me or not, the said bridge pounds itself due to the weight of the passers-by. So camera-shake is inevitable if you aim your camera at the mountains from there.
See timetable for Kamikochi-bound shuttles that depart from Akandana Parking Lot:
https://www.nouhibus.co.jp/route_bus/kamikochi-line-en/
(You'll be surprised to know other like-minded people already beat you to here.)
See what I captured through my camera-finder, pre-covid, in one *early* morning during Golden Week:
https://www.flickr.com/photos/161165029@N03/28018195968/in/album-72157696592338045/
Have a blast!