when I try to add thickness to my model, it doesn’t actually add to the geometry - it just seems to create a ‘ghost’ figure of the thickness and it will only let me select polys from the original mesh
It does add geometry. It duplicates the surfaces of your selected object, offsets the duplicated surface to the inside or outside (user selectable) of the object surface, and bridges the 2 surfaces to create a new “form”.
The “ghost” effect is caused by your transparency being turned on. Turn it off to see the figure as a solid.
With 2 surfaces, one in the foreground and one behind it, you can only select polygons on the one in the foreground. Turn the scene so that you’re looking at the other side of the object to select the other faces.
If your “thickened” object forms a pocket that makes it difficult to access surfaces inside it, you can select the outer faces and hide them, leaving the inner faces exposed to work on. If you have to do that repeatedly, you can select the outside faces and assign them to a shading domain. Then you can select that domain (which highlights the faces) and hide it. Then you can assign the “unassigned faces” to a second domain. Thereafter you can hide/show whichever domain faces you want to work on.
PS: Just an aside, but DAZ’s genius spell checker flagged the word “selectable”. Try it and see the ludicrous “corrections” it suggests