Hey Pilou,

Instant mesh is really not great at this quad recontruction job, I usually get way better result using Quadremesher : https://exoside.com/
While still producing cage mesh a tad complex for the Subd to Nurbs, I think it does a decent job at recreating the polyflow.

I think Rhino does have a version of that same quadremesher now too for that kind of complex mesh to Nurbs conversion.