The honest rule I give everyone: pick one direction a day, not three
People arrive with a list of nine places and a two-day window, and they try to staple Hallstatt to Berchtesgaden to a lake in one loop. It does not work. The mountains are in the way, the roads are slow on purpose, and you spend the day in the car instead of by the water. So I split the region the way I think about it: south into the gorges and the ice cave, east into the lakes, and across the Bavarian border to the Watzmann. Each one is a real day. Do one well and come back for the rest.
None of these roads are fast, and that is the point. The valley out of Salzburg narrows within ten minutes, the limestone closes in, and by the time you reach the gorges the speed limit stops mattering because you would not want to go faster anyway. I tell people to roll the windows down at the first tunnel after Golling. The temperature drops, the river gets loud, and you understand why the salt traders fought over this corridor for two thousand years. The Watzmann main summit stands at 2,713 meters, the Konigssee runs 190 meters deep, and the ice cave holds minus 2 degrees in July. The mountains do the talking. You just have to give them the time.