Here is a one-minute screen capture as well as the scene file.
The idea is to use the Landscape object in spherical mode and have four similar objects under the Cloner. While set to blend, you get a lot of variations out of it. I used the Shape>Ellipsoid here, as a moving mesh would have been a long calculation.
With the MoGraph Select option, select the clones that are not useful, and with the Hide Selected art-direction is an easy task.
To your questions:
The Connect object reproduces all children of it as a single new dataset. When I use the Current State To Object, the Connect becomes a Null, and the children polygon object[s].
Yes, I used eight cubes in the first example. But since you like to have more gravel-like results, I dropped the ideas with cubes.
I hope that works for you, and if there is a question, please never hesitate to ask.
I have not included the HDRi map, as you might have your own or pick one from the Content Browser.
All the best, stay safe, stay healthy.