This reminded me of a post awhile back.
There are two general ways that I see to solve the problem of carving on these compound, maybe irregular curves.
The first way would be to create a 3D model and project onto 3D model but how to create the model? You could use a laser scanner, not exactly cheap but getting more affordable all the time or try photogrammetry software like Meshroom, free and open source, which creates a model from a bunch of photos that you take.
The other way would be called ‘auto leveling’ or ‘surface mapping’ where a program probes the work piece in a grid pattern and then adjusts the G-code based on the curved surface that was mapped out in the probing step. This short video shows the surface mapping in Estlcam which is not free. I think that cncjs also has this feature. I have also found OpenCNCPilot that has this feature.
In order to do the auto leveling/surface mapping method you will need some kind of probe. A regular low cost probe can be used but you have to make the surface of the object electrically conductive. I’ve seen tin foil used for this then one wire of the probe circuit is connected to the tin foil and the other to the bit. There are also more expensive probes called 3D touch probes which don’t require the surface to be conductive because they basically have internal switches that activate by pressure or movement on the end of the probe.
That’s what I think I know about what you are trying to achieve. I hope that it at least you won’t be googling blindly now.