China temperatures

For Yunnan, Dali (Erhai Lake) and Shangri-la are the best to avoid summer heat. Among the cities you mentioned, Lijiang is suggested.

And Hailuogou in Garze and Bipenggou in Aba are suggested in Sichuan. By contrast, Jiuzhaigou is cooler than Chengdu.

It is cold in whole Tibet, and the temp in Lhasa may be a little higher. So have fun in downtown area.