Don't ride I-5, soooo boring, super hot, and yes it continues to be a high-speed heavy truck-traffic route the entire way. Actually - not even sure it's legal to ride I-5.

Sierra Cascades route would be better than the coast. If you've toured before, you know how much more important the roads are than the cities. "It's the journey not the destination" - right? So, maybe re-think that "have to ride to Seattle" think if you don't want to ride in the mountains.