Congratulations on your upcoming retirement. If you are anything like me, you will never look back!
We just returned from a 9 week trip out to the west coast and we learned a lot about reservations. We reserved almost the entire trip and it was a mistake. There were many places we would have liked to stay longer but could not do so. We camped in mostly state and national parks and traveling in the fall, most has sites open - even Yosemite! I would make reservations for weekend in popular places and take your chances on the other nights. Do what others suggested and be flexible.