I live in and ride around the Portland area, both sides of the river. The route you mapped while doable is not one that I'd ride, especially on a weekday. The roads you've mapped are very narrow, lots and lots of tight curves as well as up and downs. None of that is bad in and of itself, however, they are also roads that see little to no bike traffic, for a reason. Drivers are not "kind" up that way because they don't expect to fly around a curve and there's a bike. The farther east of I-5 you go is where you find excellent cycling roads but the trouble with that is small "bumps" like Mt St Helens being in the way. If I "had" to ride back to Seattle I'd use Hwy 30 as my first option. Wide breakdown lanes to ride in and lots of bicycling usage which means the "locals" aren't surprised.
Hwy 30 is certainly not the prettiest route but in my mind it is the safest.