Reconstruction in parts for huge reconstruction

I’m reconstructing a very large canyon using 2000 images.

The normal detail reconstruction is over 600M tri and hundreds of parts.

When I simplify to 30M tri, I would like it to simplify in parts with a max tri count of 5M.  I set this maximum in the Reconstruction Settings under Max Vertices Count per Part.

When I simplify though, it creates a 30M singleton.  

I tried exporting the mesh to obj with Save Mesh by Parts to “True”, but it remains a singleton.

Other than manually creating a series of reconstruction regions and filtering the pieces into different parts, is there a way to force the split when I simplify or export?