China 2 week trip from HK to Shanghai questions

Hello everyone! Planning a 2 week trip in China the second half of August. We are arriving at Hong Kong and back to Europe 2 weeks later from Shanghai.

1st question: sinc HK is a special territory, do we get checked by customs a second time after leaving HK for mainland china (to Guilin)? Or is it the same as if we had arrived at aby other chinese city?

2nd question: We plan to spend most if not all our 1st week in Guilin/Yangshuo/Tongren and Zhangjiajie area. After that, we'll have about 5 days left. We prefer outdoors/landscape. Was thinking of Sichuan but I read it's lengthy to travel around there (Litan county, danba, tagong). Do you have any recommendation for 5days? Can be train or flight from Zhangjiajie.

Thank you!