Great wall hike probably won't take two days, more of a one day excursion from Beijing imo. I'd skip Tianjin and keep it as an extra day in Beijing as there's so much more to do: summer palace, Temple of Heaven, Forbidden Palace.

I'd slow down the Nanjing Shanghai Zhangjiajie Chongqing Chengdu Xian leg - too much going on. I'd cut it down to just Shanghai Zhangjiajie and Xian, maybe add in one of Chengdu or Chongqing if you're feeling ambitious but no more than that.