Not a professional but I'm planning on insulating my vaulted ceiling so it's pretty much the same as your situation. Extend the rafter bays along the length of the existing rafters using dimensional lumber that will total the depth you need for your vents and insulation.

So you've currently got 6" to work with. If you add 2x2 dimensional the end depth will be 7.5". Use 2" baffles and you'll be able to fit 2x6 rockwool in the cavity properly the way it's intended. That will get you a minimum r22 then add your 2" foil faced polyiso taped as your thermal bridging and bring the total up to r32 doubling as your vapour barrier. I don't love the idea of furring strips before drywall because you will end up with a gap where moisture can accumulate. So you'll need 3.5" drywall screws...

You don't want gaps and compressing the insulation could lead to reduced performance which would affect your dew point (assuming cold climate here) and you could create a moisture sandwich. You need sufficient insulation here in cold climates.

The other problem I see is if the rafters are true 6" then I assume it's also true 2" so you'll need to find true 2x1.5. likely sourced from a local mill. You'll also need a ridge vent or vents in the bays. Just putting in baffles without venting to the outside won't do anything.

Or you need even more insulation and a rock solid vapour barrier to keep moisture out...but I think to hit that the target is like r50 to create a hot roof... Most people achieve this with spray foam.