Should add a couple of things:

1. If you take the road along the Joe, then up to Gold Pass, and then down to St. Regis, I would avoid weekends. I went on a Monday and camped at Tin Pan Flats up river from Avery. There were leftover hang tags at nearly every site there and at the campground a little bit up river. That suggests busy weekends. And that was only mid-June. The Joe is a popular river. I'd bet there is more traffic later in the summer.

2. I recently read a blog written by a couple who rode ID-5 east to get to Avery. They said there was an 8 mile stretch that was terrifying, with lgging trucks and generally fast traffic on a twisty, shoulderless road. They seriously considered trying to hitch a ride. IIRC, that was east of Hayburn State Park.

Some photos.

The NorPac Trail west of Saltese, MT


East slope of Gold Pass out of St. Regis (You would be going downhill)


East slope of Gold Pass out of St. Regis


East slope of Gold Pass out of St. Regis


Gold Creek coming down from the pass on the ID side.


Riding along the Joe towards Avery


My campsite at Tin Pan Flats