New metal roof in the Vermont snow belt

Need some advice on exactly what to do here.
1960s ski house. Has an unconventional cathedral ceiling. 4x6 rafters, 4’ on center with (I believe) 2x10 tongue and groove decking.
4:12 pitch.
There is “two” roofs on now.
The original, then an air space ( I believe was an afterthought to prevent ice damns.) 2x4/ply and shingle on top of the tongue and groove decking.
I’ll be installing exposed fastener sheets.
The inside ceiling is insulated with only 1” styrofoam. But will have some type of board insulation as well.
How should I do this?
I plan to tear of down to the original decking, water and ice the eaves, then 30 lb the rest. I’ll need a vented airspace I imagine?
This is a low budget project.
Any advice would be great. Should I insulate above the deck? Special underlay?
Tough to find a solid answer.

Is the foam to the inside just exposed here or covered? Just wondering how the T&G is finished.

I would go in the same direction they were going, but with some above deck insulation ventilation combo.

Remove the additional layer of decking, cover the entire roof in a layer of foam, add additional battens and purlins here to create a vent space, and then vent continuously from eave to ridge.

Definitely not the easiest application, but it will work. It will add to the rake board height so you will need to cover that on the sides with trim accommodations.

Thanks for the quick response!
The inside insulation is exposed. I plan on 4” of board and then a tongue and groove under that for visual appeal.
On the deck:
Should I apply felt ended the insulation? Or a specific type of synthetic perhaps.
Then something above the foam? To allow moisture run off?
I’m I full on amateur with metal!
Thanks again

Thanks Eric, I just realized I never responded.
I ended up using a breathable membrane, then I installed battens and purlins as advised. Looked great untill the roofing company did their work!
Now I’m back with a new thread looking for professional advice!
Again, apologies for the late response, and thanks for the great suggestions.