I think that’s just a software issue. I guess if your Librem 5 was a bit haywire before you docked then it may well be sensible to reflash before doing much testing with the Nexdock XL.
Assuming that video is working to the dock then the bare minimum is to check lsusb output.
On my lapdock, NB: not a Nexdock, I had to do some gsettings magic to get the touchscreen to work. Did the touchscreen work out-of-the-box for you?