Thanks for the suggestion.
Hierarchical layout AFAIU assumes the rectangular shaped group nodes and lays out their child nodes accordingly, whereas in my case the group nodes are elliptically shaped.
The circular layout should, in principle, provide what I want, but I don't understand why 'disk' layout doesn't put my isolated nodes into the interior of the disk, only to its boundary (and how it is then different from the circular) and why in 'organic disk' layout the interior of the disk is only sparsely filled.