5/8" decking applied to top of rafters.
self healing roofing membrane applied to that.
4" to 6" of polyiso foam insulation - 4x8 sheets - about $.50 a board foot.
1/2" decking on top of foam with long screws and washers going through 1/2" decking and foam into 5/8" decking
Whatever roof you prefer on top of that.
The top layer of 1/2" decking can be replaced as needed when the roofing is replaced.