I'm local. Been doing that route forever.
Heading south on I-5 from B.C., exit 208 east to Arlington. South on WA-9.
Two choices: (1) Stay on South 9 to WA-522 to south I-5 to I-90 east at Factoria, which is south of Bellevue, and head east over Snoqualmies Pass. Depending upon time of day the route from 522 to 90 can be horrendous.
OR
(2) Stay on WA-9 and take east US-2 just south of Frontier Village/Lake Stevens. At Monroe take south WA-203 through the beautiful, rural Snoqualmies Valley. Pick up I-90 east either at Fall City or at the Snoqualmies Parkway or at North Bend. This is the way we go when we figure I-405 is going to be slammed from 522 to 90. It is a good 2 lane and easy on the eye and the nerves.
Not sure what new bridge Paulj is referring to on WA-9 over the Pilchuck River, unless it is the new bridge that was built over Pilchuck Creek about 4-5 miles north of Arlington. WA-9 does not cross the Pilchuck River. The new bridge described is not on either of the routes I am recommending.