Insulating and venting a cathedral ceiling
You could install a continuous roof ridge vent and continuous roof soffit vents, and leave a 2 inch air space between the underside of the roof sheathing and the top of the ceiling insulation. You don't have a lot of space to develop a large "R" value with your set up. You could maximize your "R" value,while minimizing the height of the insulation by using a type of insulation with a higher "R" value per inch, such as urethane.
The venting of each rafter space in a cathedral ceilings is required by the International Residential code (IRC).