I cannot help with the start date, but will suggest a side trip to Yosemite if you have not been there. I did just that on my Pacific Coast ride and would do it again without doubt. It takes a couple of days each way across the Central Valley and a minimum of two days in the park. Off season will be nice there.
Yosemite is awesome. If you go there it is worth spending several days to a week hiking.
That said... It is a pretty big side trip from the coast and I would be inclined to visit it as part of a different trip that would possibly also include Sequoia.
Also be aware that Yosemite is not the most cycle touring friendly place in the world. There is road work for construction that I think will be going on for a couple years 24/7. The construction zone is pretty long too. They make no provision for shuttling bikes so it is a bit of a hassle to get over the long section of construction.
Also be aware that unless you have reservations you will probably need to camp one night in the North Pines Backpackers Camp and then move to Camp 4. They allow touring cyclists one night at North Pines and then you either need get up at the crack of dawn to get in line for a site at Camp 4 or to move on. That is what we did, but be aware that even though we got up at 4 AM we were not first in line at Camp 4 and by 8 AM there were maybe 50-60 people in line.
Off season may be less booked at Yosemite than when we were there though.