I havent actually hunted the later part of september eastside. So dont have a ton of info on how vocal the bulls are then. My assumption would be similar to yours in that regard.
I always go opening weekend.... I havent killed a bull yet. But most of it is due to logistics and not nessacarilly a lack of elk encounters. U already have the logistics part taken care of it sounds like in the way of a drop camp.
My experience is that how key elk vocalization is to a hunt being "good" depends on several factors weather, TERRAIN, the hunters goals, and hunter pressure. The area I hunt has no pressure in the times I am going. The terrain lends itself fairly suitable to spot and stalk, and bulls are responsive to cow calls. Herds tend to be smaller groups and bigger bulls seem to secumb to cow calls quicker. Bulls bugle but its not the frenzy that is the later part of september. But if your in an area with more pressure, bulls may clam up, and if the terrain is such that alternate methods of hunting can be successful you could be setting yourself up for disapointment. In the end alot will depend on what your comfortable with.
