Meandering thoughts:
Working in residential renovation for decades one of the finest forms of thermal exchange insulation we found is sprayed in Iso foam.
It settles in at about 6.9 R per inch of thickness. Three other benefits are, a total block for insects, total block for tiny pin hole leaks of cold air in strong winter winds plus tremendous sound attenuation.
BTW, one of John Armstrong's cautions is the factor that going in we have a tendency to lie to ourselves as to the real construction cost, ongoing energy demand and tax expenses.
Plus upon eventual resale of the property a large accessory building limits the potential pool of prospects.
The most effective foot prints, IME, has been more of a rectangle than a square. Considering the over all effort here the net effect of the layout; after reviewing the vehicle provision, the wall displays and the classic layout space, could fit into a much less expensive/extensive building.
Maybe review your target. Are you attempting to construct a museum type of introduction for visitors to the world of model railroading with your personal layout being one of the aspects or is this effort primarily to house a layout? It's a good size but it can get real small real fast with a full plate of expectations.
Unless you can freeze the aging process and lock in a healthy body it may be wise to consider a non duck under platform.
