Maybe look at exterior insulation and a double deck setup. The bottom of the deck, just go with ply/roofing grade OSB or ZIP roofing. Tape and flash as normal. Drop in 4 inches of your preferred insulation. Rigid EPS, Rockwood, PolyIso, etc. Cap the assembly off with ZIP roofing. Again, tape and flash as normal. Essentially you're building a SIPS-like structure. Furring strips above that for the metal roofing. Then just insulate inside with whatever takes your fancy. Foam, Rockwool, blown-in, etc.
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