My friend Dave and I made a run out to Carrizo Plain National Monument in the springtime as a scouting run to see if the super-bloom had started yet. Starting and ending in San Francisco as an overnight ride, there is a lot of somewhat boring slab (highway) to tackle before the roads transition from meh to excellent.
Carrizo Plain National Monument is free to enter and camp out at. It's sort of a well-kept secret that's also public knowledge, and can really capture the desert charm you may be after. The roads are generally really easy to navigate, and the toughest sections are only when you start to veer off the plain. Sections of sand and mud will appear depending on where you ride, but it's all doable, even on our big BMWs.
Make sure to stop in Parkfield on your way down. It's barely even a town, and it's got some of the best burgers you will eat in California along with some interesting folks to meet. https://www.yelp.com/biz/parkfield-cafe-parkfield
If you go in the spring time or right after it rains, be prepared to battle some mud.