Hwy 12 is definetely on my list. As much as I hate to, I am coming to the conclusion to bypass California. Thanks for yourBecause you have mentioned wanting to go to Zion, you should also visit Bryce Canyon which is just up the road. And, as long as you are in the area, riding Highway 12 through Grand Escalante National Monument, either from Torrey to Bryce or vice versa, should be on the list. It is spectacular.
The Pacific coast is amazing, but realistically, with only three weeks to ride, you might want to forget about riding all the way to the west coast and, instead, concentrate on the Rockies, from Montana south. Just a thought.
suggestions.