Good idea adding numbers. Maybe to the more obscure ones, that can not be easily identified.
You can coat XPS with resin to harden the surface. PVA and sand, woodfiller etc. also add a nice protective shell. I do agree that XPS is maybe not exactly good for the environment. That said, I use any off-cuts etc. very little goes into the recycling bin. I did not come up with a good alternative, that is equally light weight and versatile.
The stone texturing is also a good idea. A piece of bunched up tin foil works well, too.