With the help of a number of people here, I ended up with this Macro:
! -Export Pause GeometryOnly=Yes SaveTextures=Yes SaveNotes=No “/Users/akash/Zbrush Work/FromRhino.obj” VertexWelding=Welded YUp=Yes Enter DetailedOptions AdvancedOptions Angle=0 AspectRatio=6 Distance=0.0005 Density=1.0 Grid=0 MaxEdgeLength=0 MinEdgeLength=0.0001 EnterEnd
And if it was a subD object then it can usually be directly worked on in Zbrush. It comes in very cleanly as a fairly dense quads. [Just hit the command+D a number of times to add up devision levels]
for the way back to Rhino. the best way I found by now, is to send a decimated version. [perhaps a million poly at most] Lock it! and make no attempt to edit that mesh in Rhino.
I’d imagine a stronger computer will be more responsive…? My old machine can manage a 10 mill poly object in ZB easily. Yet in Rhino 1 mil poly is already too much for it.
there for the ZB mesh in rhino is used locked while creating additional parts. Then these parts are exported to ZB for Live boolean, and the sculpting work.
The finished part decimated and exported as .STL, I’ll again open in Rhino to verify all dimensions remains as intended.
Would Love an easier quicker way, but I think the main thing here is about how Rhino is poorly handling high poly mesh.
and the ZB with will be to be able to import rhino Curves converting them to ZB curves [to use with the power of Curve brushes].
with best regards