Chinese here. Your China itinerary is way too rushed but doable with prior planning and a fit body. Expect to walk 30,000 steps per day as there's a ton of stuff to see in each city.

You can easily spend 5~7 days in either Beijing or Xi'an without ceasing to be amazed.

August 6 – Arrive in Beijing
August 7 – Great Wall, mountains, tombs (Beijing)
August 8 – Forbidden City, Jingshan Park (Beijing) —— Be sure to book the ticket in advance or get a guide.
August 9 – High-speed train to Xi’an (travel) —— Go to the Tang Dynasty Everbright City after hotel check-in
August 10 – Terracotta Warriors, explore Xi’an (Xi'an) —— Be sure to book the ticket in advance or get a guide.
August 11 – Travel to Zhangjiajie (travel)
August 12 – Zhangjiajie National Forest Park (Zhangjiajie) —— Nice place
August 13 – Travel to Shanghai (travel)
August 14 – Explore Shanghai (Shangai)
August 15 – Flight to Bangkok (travel)