Hi planning to visit Kashgar in early September. What's the best plane / train route from San Fran

You are suggested to take a flight to Shanghai first, and then transfer to the flight to Urumqi. After arriving, you can take the flight to Kashgar.
The temperature may range from 15C to 25C at that time. It will be warm and comfortable. Generally speaking, the weather condition will be good.
However, I am afraid that it won't be easy for you to find a driver to take you around for 10 days.