I would like to clarify... What I meant to say is that you can import some arbitrary STL into Fusion 360 and generate your gcode. Fusion can generate parallel toolpaths just fine and you don't even have to convert your STL to BRep. It easily produces toolpaths with smoothly varying Z value. And yes, it does work for basreliefs, I tested it myself :) Also, you don't really have to design your 3D model in Fusion, you can use Blender or any other editor you like.