When I do lithophanes I use Candlestone and three passes to get the best possible detail - 0.25", 0.125", 0.046". A 5x7 takes about 3 hours, including the tool changes.
I use BobCAD-CAM plus Art so I’m using industrial embossing CAM. I use a adaptive, High Speed Machining (HSM) rough then an adaptive finish pass and finish up with REST milling finish.
I’ve also used MeshCAM and the results are quite good.