I would get up through there before it gets too late in Septembet. Late September and early October the snow could easily fly up in that high country.
The main road is not too bad, but most of the spurs are truly primitive roads and will require 4-wheel low.
My son and I drove up form the Boise area (Emmett) and really, really enjoyed ourselves...just wished we had had a whole week to spend...it is more than deserving of that much time.
We just came back from the Tetons - but will remember this for the next trip! (Last summer we did about 200 miles on dirt roads on our way to Las Vegas!)
I did a job out of Salmon, Idaho and we blew two tires on the company vehicle in one day. We drove a lot slower after that, but ended up buying a whole set of tires (used) so we would be able to drop the flats at the shop and go on up to work. Lots of sharp shale rock in the area which ends up in/on the roads. Beautiful country - thanks for the photos.