Additional suggestion for a route that is both pleasant and avoids Portland. I've travelled this route many, many times in the 9 years I lived near Seaside, OR and would be the route I'd go, no question.
North on Hwy 101. At Astoria, get in left lane to turn onto the Megler Bridge into Wa. When in WA, right to Hwy 401. Left at Hwy 4 then north right back onto Hwy 101 towards Aberdeen. Before arriving into Aberdeen, right onto Hwy 107 which is a cut-off to heading east. Then East on Hwy 12 into Olympia. North on I5 to continue on to your normal mapping to get to Leavenworth, WA.