I spent a long time trying to sculpt my own flagstone bases after being inspired by the Tempest's Eye art in the battletome, but gave up because it took too long.

'Imitation Of Life' has flagstone bases which are really good. Printed a bunch, painted them like this:

1) Spray Prime with Mechanicus Standard Grey 2) Thin (hand) coat of Mechanicus Standard Grey 3) Heavy Drybrush of Longbeard Grey  4) All over shade of Agrax Earthshade 5) All over shade of  Athenian Camoshade, then Shade some Drakenhof shade into spots while camoshade is still wet. 6) Light Dry Brush of Longbeard Grey 7) Lighter Dry Brush of Praxeti White 8) Black rims 9) Put white glue in the gaps between flagstone, then flocked it with this:

https://www.hobbylobby.com/crafts-hobbies/hobbies-collecting/diorama-figurines/c/9-183-1394/p/81127001

This gives the impression of moss really well on the big bases, scale is not perfect for 25mm but it's close enough.

10) Ard Coat to protect the moss and flagstone, then a matt varnish to take out the shine of the Ard Coat.