395/44/89 diagonally up to Shasta is direct without major grades. It mostly stays in the 5000/4000 ft level, same as Reno.
I5 across the OR board does have one modest pass. My only complaint was that I felt like I was play hop-scotch with all the trucks.
OR42 is one of the lower roads across the coastal mountains, and is the most direct route from the central corridor to the south coast and Coos bay.
I think all the other suggestions will be longer and involve more grades. Crater Lake is a mountain top.
US97 from Weed north is relatively flat, staying at the 4000 level past Bend. From there to the Cascade passes the climb is minor (especially OR58), but grades down to near sea level are long.
Another option is to cross the Sierras on I80 or CA20, and then 20 across the coast range to US101. Then its redwoods and coast all the way north.