Happy to help how I can. I would love to see this happen.
Fall is end of August early September. Subtract from that as you go north. It's also wet. Possible snow as you go north. In south central you can push that timeline till October. Fall is one of my favorite times of year. Remember hunting seasons open then and the amount of company you will have in the backcountry goes way up. On the plus side the trout fishing peaks at this time.
If I had to put money on weather, I'd go with solstice. I know many cant make it that soon, but really many of us can't make that kind of trip regardless of lead time, so I'm not sure that should be the driving force in your decisions. I imagine that with some notable exceptions those who both can and would commit to this will find a way even with short notice.
If you really had to push it back I would look at the last week of July and first week(s) of August. You gain 4-6 weeks lead time and are still in before the cooler weather and hunting season kick off (depending on the game unit that is around August 15) It does put you late for the red salmon and perhaps a touch early for the silvers.