I decided to switch to an approach which treated only the large details of each rock asset as unique. the smaller details of the rock (the grain, lichen, cracks etc) would be added with a tiling texture (whilst hoping nobody notices the seams!)
In addition to allowing me to downscale my asset's texture to 512 (although I now needed a couple more textures, but these would be shared across all my rock assets), it allowed much sharper detail, and I could make each rock unique using the object's position and even scale the detail with the scale of the rock.
I also threw in vertex painting of moss to allow me to make the rocks near the waterfall a bit mossy.
I used custom lighting to allow me to give more light variation in the lit area, and a topdown light bias in the indirectly-lit areas.
I threw a few of them together to see what I could get with just this one rock asset. A good result, though I expect I'll need one or two more rock pieces for cliffs at least. And the moss is a bit nuts at the minute. Easily tweaked later!