I contacted the makers of Affinity Designer & inquired if a SDK is available for their “.afdesign” file format. Unfortunately, their developers don’t have plans to make the file format public. They have listed .dxf / .dwg file support as a requested feature on their forum but unsure if / when it would be implemented.
Printing a vector Adobe PDF from Rhino seems to convert smooth curves to interpolated curvature by using straight line segments. I have tried exporting .dxf files from Rhino (both with and without the ConvertToBeziers command on needed geometry), but in my experience, the curvature is often distorted & not interpreted correctly by other programs like Inkscape or QCAD upon import. I could then export the geometry from either of those programs as a .svg file for further editing, but it’s no good if the geometry is inaccurate to begin with.
As other users have suggested, please consider adding .svg import / export functionality in Rhino for better (and hopefully more accurate) compatibility with non-Illustrator vector applications. I’d be curious to see if the .svg curvature information from Rhino is retained when opened in a program like Affinity Designer or Inkscape. Thanks for listening.