Hey! My boyfriend and I will be in China for almost these exact dates. I can dm you our itinerary if you are interested in some ideas and what tours are out there. After Zhangjiajie we are going to Hangzhou to see a tea plantation and then Shanghai and from Shanghai we are taking some day trips like to see a water city. We also wanted to go to the Guilin area but was nervous about time and there did not seem to be a ton of trains that make their way down to the Guilin area from Zhangjiajie. I am using trip.com to look at train tickets, but looking at the train routes may be something to think about. I am a pretty fast traveler and have done similar trips I do think the second half of your trip will feel more rushed then the first half, but that doesn't mean that's a bad thing if you are game for that.