You seem to have pretty good knowledge. I have seen some spray insulation kits on line but I have never used any. My stepdad has a metal building and he put plywood 4x8 sheets around the lower 8' and that alone made a huge difference. My best guess would be a combination. Thin foam board against the metal, pink insulation over it then plywood. If money were no object I think spray foam. I am considering a metal building, barn house at some point so I hope to learn the best insulation for these as well.