I would actually suggest the coast for large wide shoulderin most places and also for the scenery. Even if you're not one of those tourist, please your eyes and keep distracted a bit while riding your 300 km per day.It's a bit hilly at times but very doable, especially if you travel light. Astoria visitor center give away a free cycling map of the Oregon Coast. This has been most helpful to me. Accomodation can be sketchy in certain areas of California, so make sure you plan ahead, unless you can ride all night too!!! Be ready for some cool mornings, and for fog in Washington state and Oregon, at any time of year. Makes for cold riding and cool nights should you camp.

I am most impressed with your choice of touring, (am the 65 pounds load, 100-170 km per day type) I think I understand your philosophy a bit coming from an endurance and endorphins background .Cheers and good luck. Kati