Hi, I am looking to go from Guilin to Zhangjiajie on July 5.

It's not necessary for you to take long-distance bus from CS.
There are also several trains to Zhangjiajie that depart from CS Train Staion. So you don't have to experience a complicated transfer.

The available trains only leave at 10pm and are 11 hours long - is this still worth it?

Oh, from CS to Zhangjiajie, taking a long-distance bus is a good choice for you.
The duration is about 5 hours.
Most buses of this route depart from CS West Bus Station.
So after arriving at the railway station, you can take subway line 2 to Wangchengpo Station, Exit 1B. Then walk south around 350 yards to arrive at the bus station.
It's not a complicated transfer. Don't worry.

Thanks!