They just replaced the Pilchuck River bridge on WA9, eliminating one narrow curve.
South of Arlington 9 is getting busy, especially in the Lake Stevens area. Also once it climbs the hill south of Snohomish, though they have put a lot of work into streamlining flow (as far as 522). WSDOT has a number of webcams on 9.
I don't see the point to jog back to 405 in Woodenville. 5 and 405 north of there isn't any worse than 405 to the south.
To stay out in the country, I'd go east to 203 (Duval). You could even pick that up in Monroe (or with backroads via Granite Falls).