Thanks for the thoughtful responses.
All of my terrain carving will be at scales like 1/8" = 1’ where GIS data isn’t specific enough. The smooth terrain models I’ll be making will be built in Rhino directly from surveyor’s CAD and architectural plans, altered to include things like paths, roads, driveways, building sites, retaining walls etc. In the past I have made these features out of layers of cardstock, laser cut when I had access to a laser, and cut by hand when I didn’t. Being able to carve smooth terrain will allow me to build more sophisticated models with less time investment.
It looks like FreeMILL operates inside Rhino and exports G-Code as an .nc file through a Shapeoko post processor.
In that workflow is my last step using a G-Code sender to operate the machine? Is there a preferred program?