Ok here is my basic PBR setup.
I may be wrong but my understanding of PBR work-flow is that the images contain the relevant data for Albido (defuse colour) specular and roughness and that therefore these should not have their values changed as they are plugged through Terragen's shader network.
Therefore the albedo goes in the defuse channel and that channel colour is set to white 1 (see ScreenCap1).
The Reflectivity image is on the Specular tab so we put the specular image in there and set its value top the max (0.
. The image is actually very dark, almost black, so setting the value to 0.8 will still only give a very low reflectance.
Similarly the roughness image goes in the roughness channel with the value set to 1. The image is very light in this case which makes sense for rough granite. Setting the value to 1 means you will be using all of the values set in the image to scatter the light.
Displacement is NOT used since Terragen dose not support it on imported objects.
I have also created a surface layer for the ground using the rm4kshp_4K (Old Concrete) which is also free on the MegaScans site.
Obviously none of these settings are locked and you would need to tweak them to your own personal needs.
You can also make your own versions of the MegaScan files by layering them in Photoshop (or similar). Try using the cavity map to accentuate the albedo and specular maps for instance.
I hope this helps
The attached file does not include the MegaScan Assets, they can be downloaded for free from the MegaScan site.
Scatter Rocks (they will have to be separated into individual rocks and saved out as obj's from your modelling application of choice) here: https://quixel.com/assets/phxuF
Old Conctete ground texture here: https://quixel.com/assets/rm4kshp0