In the end a 75-100mm layer of road base was spread across the site, resulting in a flat, smooth surface, with just a gentle 30mm or so slope across the pad for drainage.
The road base will compress over time and provide a great flat open area, stable under foot, for setting up extra telescopes to enjoy star gazing and some visual observing.
Road base is also light in colour and has low thermal mass, so doesn't absorb too much heat, which is a good medium to have around your observatory to preserve the seeing.