You can replace concrete texture with gravel texture and change the material to sand (or what ever) in the Layers library.
You can also first clone the concrete sline and name the clone to "Gravel Cart Path" for example. You can add as many splines/layers as you like.
I am no layers library expert, but:
yes, yes, yes and I don't know what the max is but I've had as many as 15 layers without problem. I've always used the CF physical materials and there is no gravel provided in that list.
Unlike adding painted textures which has caused me some pain, the layers library has been kind to me
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