I'll second your vote for the Cascade Hwy. I ride it every time I get to Washington. I usually start over at Hwy 31, having entered your state from Salmo BC. It's definitely a destination ride. The road is great, the people are friendly, and the food is good. What more can you ask for?
I haven't ridden the rest of your route; but will definitely give it a shot next time I'm in your neck of the woods.