A good start you got there. I would try to add come variety to the rock density as its very uniform at the moment. Maybe have a couple more layers at different sizes and using slightly different values in your distribution shaders. I would also add a few more ground layers and dont forget that when you add a new Surface Layer it comes in with a coverage of 1, this will cover all layers before (apart from its breakup, height and slope restrictions), so drop your coverage down somewhat.
Its basically a trade off between subtly of detail (lots of layers) and speed of render (fewer layers), err put very simply (lol)
As regards your slow PC there are a couple of things you can do to speed things up. The volumetric clouds and water layer will eat up most of your render time, consider if they add to your scene or not (In my opinion the water at present dose not, didn't even see it at first). Remember there is little point in rendering at above 0.75 detail level, it will take forever and not be particularly visible. You can also take the GI Relative Detail and GI Sample Quality down from their default 2 to 1 or 0 and un-check the tickbox "GI surface details". While you are building your scene you can render at even lower quality and don't forget to use the crop region to focus in on details.
Keep at it and good rendering
Richard