6) low traffic being maybe 1 motor vehicle every 10 minutes
This one's going to be tough to satisfy on any road that's likely to be an enjoyable ride anywhere in the central Puget Sound area, unless you can ride at night or early on Sunday morning. Even sticking to mostly residential streets I can't cut out enough traffic to meet this standard during the day. Lots more rides open up if you can tolerate more like 1 car every 2 to 3 minutes.
For the 10 minutes per vehicle average, I'd try going over to rural areas of eastern Washington like the posters above have suggested- or the back roads of southwestern Washington, or rural Skagit and Whatcom counties. Or try the MUPS that they've mentioned.
One road like this that I really enjoyed riding last summer was the Rock Island Grade road, which starts off SR 28 about 8 miles SE of Wenatchee. It's a great workout climb with nice views at the top, and I think I was passed by maybe a dozen cars during my whole ride of 2 hours. It's unpaved, but doable on a road bike with wide tires.