I-5, I could not hear it. From Red Bluff I joined SR 36 then took a number of local roads (Beegum to McCoy Rd to Hooker Creek Road to Bowman Road to Draper Road to Main Street in Cottonwood to Locust Road to SR 273) to travel to Redding. I had originally considered taking 36 to A6 to 44 as you are planning, but decided to use 299 from Redding. The challenge with the route I selected from Red Bluff was the McCoy Road section. This was gravel for about three miles.
299 from Redding was not bad. Yes, shoulders are minimal but generally drivers were considerate. I took 299 to Bieber. At Bieber, rather than continue on 299 to U.S. 395 to U.S. 20 in Oregon, I took I took the Lookout Hackamore road out of Bieber. This is a very lightly traveled road through ranch land and the Modoc National Forest. Excellent road. This road connected to SR 139 to Tulelake. A more traveled road, but with a good shoulder.
I continued on SR 139 to Oregon 39 to Oregon 140 to U.S. 97 to Bend. Good shoulders in Oregon. Traffic is not bad, even on U. S 97, which has, for the most part, excellent shoulders. I decided against U.S 395 because there is a whole lot of nothing on that route.
From Bend eastbound I decided against U.S 20 because, again, there isn't much on that route. I would suggest U.S 26 eastbound to pick up your route to Montana.