In general northern Idaho highways often have poor shoulders for cycling.
In particular the steep drop and climb to the Kootenay River at Bonners Ferry tends to have high traffic and poor shoulders and no real safe place to be on a bike.
Side trips?
One obvious and easy one is that from Ione, WA you can cross the river and ride on the quiet Leclerc road all the way to Newport, WA (well technically just E of Newport in Idaho). There are some forest service campgrounds on this road, but no other facilities. Near Usk, WA you can cross over and there is a store a mile or two out of your way.
South of Nelson, BC you can follow the (sometimes rough) Great Northern Rail Trail most of the way to Salmo, BC.
A side trip to Priest Lake is worthwhile. Also, you can cut over from the Priest River valley to downtown Sandpoint on unpaved roads. The route is not difficult and you should be able to figure it out from Google Maps, but look for Big Creek Road and Baldy Mountain Road to figure out a decent route (I went this way in May of 2019). This keeps you off busy noisy highways for a good distance.
If you are extremely adventurous and brave you might consider a route that goes over the mountains to the west of Priest Lake to near Metaline Falls, WA.
Resupply options in many of the smaller towns are limited, which is something to keep in mind when doing the route. Smaller towns like Salmo and Metaline Falls and Ione in particular don't have great supermarkets. Nelson and Creston and Bonners Ferry and Newport do have decent grocery stores, though. And the small Kalispel market just north of Cusick is pretty impressive.
