I'm traveling the same route, at the same time in the opposite direction and leaving San Diego on August 15. I'm worried about the heat in eastern California and Arizona.
I spent the last 2 weeks of October in the Sierra mountains in California this past year and the weather was perfect, shorts and t-shirts during the day and light layers at night.
During the years that I lived in Western Colorado mid-September to mid-October were always the most pleasant times of the year. Typical fall weather, cool, clear, blue skies and fall colors. Enjoy the ride and hope to see you in passing.
I'm sure the weather will not be a limiting factor, though in a mountain environment expect anything and be prepared for possible adverse conditions.