The Washington part of the Northern Tier is the hardest section of the NT but by the time you get there you should be ready for it. You can do five passes in four days if you so chose. This is easier East to West as you'll already be at 2000' or so. Great scenery here as well as you are in the mountains and there is Glacier at one end and North Cascades National Park at the other end. When you reach Anacortes take the time to ride in the San Juan Islands. In fact I recommend riding the islands, ferrying to Sidney BC, riding to Victoria and taking the Coho ferry to Port Angles. Take 112 from Port Angeles instead of 101 around Lake Crescent. Definitely ride around the Olympic Peninsula. If you have the time ride into the Hoh Rainforest and spend a few days there. The Peninsula is rolling hills and is generally pleasant riding. Be prepared for rain, though it can absolutely not happen.
The Astoria Bridge will be the last thing on your route and while not a great time isn't that big of a deal.