I ended up merging them all together and then baking the layer. I don’t think it was strictly necessary, but every time I tried to paint a shadow best render shut off and it was impossible to tell what was going on. IIRC in Meats’ (first video) he explains that the “second row” of tools works best with the layer baked. I still don’t claim to understand why though.
Anyway here’s a few highlights and shadows painted on this rather gross piece. Texture on the hunk o’ flesh from lemog3d (thanks Frenchy!).

edit I just flipped back and forth between the two versions in irfanview. The shading and highlighting really make it pop.