hmmm, I can't seem to convert to Generic Solid either using v2020. I can convert to a mesh, but that should only provide a shell. Looking at the 'floors' above, I would think that getting the solid from the subdivision would be the first step. Then modeling the outside of the walls and floors as an overall 3D object using simple extrudes and Add Solids would give you something to Subtract from the generic Solid and then the interior detailing could be modeled. I would doo all of this on different design layers to keep everything lined up and to meep the individual objects easier to see as you work.
That said, the failure of the convert to generic solid seems to be a bug.
I don't think either NURBS or using Multiple extrudes will give you what you want. That said, perhaps, and maybe w=using a simply exterment, model the interior hollow, then subtract that from the 'cardboard' overall shape (having assembled that using add solids to make one object), then Co avert the result to a subdivision and while not touching the interior, work on the exterior surface?