I wondered about spray insulation or just go kingspan way. I wanted to do sips at first weight is never a problem because I don't want to build it. But I would love to do an overlander after. Cool!Having re read the OP, I think 1200 centres is not enough. Can you put wood verticals in to make the centres 600, then put noggins across so you’ve horizontal as well. I guess once you’ve got ply on you can then put DPM then batten for air gap then whatever cladding you like. You can insulate between the wood with kingspan.
I am helping a friend build an overland truck at the moment and we have made the living box from box section then infilled as above with angle not wood but principle applies. He has then put a galv steel skin on, with sika between the metal and the skin. He spray foamed the inside tthen has cladddd the interior in t&g.
I am part way through cladding my house as above with OSB, dpm, batten, cement board. So I’ve a little practice at it.