My biggest question is how you intend to "cathedral out" trusses.
You cannot cut or change trusses without an engineered solution.
There are such a thing as scissored trusses that can have a
limited cathedral ceiling, but this would very difficult after the
fact in an occupied space.
Most current thinking would maintain an air space above the
insulation and below the roof sheathing. It also common to
insulate completely with no venting. A hard decision with experts
on both sides.
Keep the whole world singing . . . .
Click to see the full signature.