Insulation options for roof, looking into spray foam or batting
You first need to decide if you want the attic inside the HVAC envelope or not. Right now it's not, there's insulation on the attic floor (ceiling of floor below) capping the area to be heated there. That's most common/traditional. Making a "hot roof" and putting insulation on the inside of the sheathing would be instead of insulation on the attic floor.

Personally, I first air sealed the gaps in the attic floor and then put 22 inches of cellulose, plus added soffit vents. Made a big difference in energy costs.