Stick with an unvented roof, and DO spray foam the roof deck with either open cell foam, or 2" of closed-cell + batts. Besides the additional R-value, using foam is better due to it's superior air-sealing, which reduces stack-effect heat losses in winter, and limits the amount of exterior air that can get into the space and condense on the outside of the ducts during muggy weather. It also reduces infiltration driven by duct-leakage and duct-system imbalances, since it puts the ducts and air handler inside the PRESSURE boundary of the building envelope.
If it can work (usually does) installing rigid foam insulation over the exterior of the roof sheathing improves performance in greater proportion than it's R-value, since unlike rafter-bay insulation, it insulates over the thermal-bridging of the rafters. This reduces ice-damming potential and boost the performance to better-than code.
There are several outlets for used 2-4" polyiso or EPS roofing foam reclaimed from commercial building re-roofing and demolition at something like 25-35% the cost of virgin stock, which takes the financial sting out of it. Held in place with 2x furring through screwed to structural roof deck 16" on center you can then put 7/16" OSB up for a nailer deck for the shingles. At the ends, put in something like Cor-A-Vent to keep the critters from setting up a condo in there and you'll have the benefits of a vented roofing for shingle-longevity, and a semi-permeable path through which the structural roof deck can dry 24/365.
You can leave the insulation in the attic floor, but if you do you may want to monitor the relative humidity in those spaces in winter. There is some (but not a huge) potential for condensation in there in winter when you split the total-R that way. Monitor the temp & humidity- if it never drops below 40F in there you're pretty safe even if the RH peaks are high, but if it's constantly ~40F with 80%+ RH in January there is at least some mold potential. In southern New England that pretty much goes away with R20 or more at the roof deck even if you have R38 or more at the attic floor. With 2x4 rafters you'll get about R10 (R13, center cavity) out of open cell foam, and if you put 2" of reclaimed polyiso brings it up to R22. With 2x6 rafters you get about R20 center-cavity out of open cell foam, but the thermal bridging reduces that to under R15, so you'd want at least an inch of exterior foam to be absolutely assured. But even without the exterior foam, if the average January temp is too low/RH too high, peeling off a few inches of the fiberglass would take care of it without a huge energy-loss penalty.
If you insulate & seal at the roof deck, insulate & seal the gable-ends with foam too, or you risk wintertime condensation at the sheathing on the gable ends. Even 3.5" of open cell foam makes a HUGE difference. While open cell foam is semi-permeable to water vapor, it's impermeable to air, and the rate of moisture transfer to the cold sheathing via vapor diffusion is orders of magnitude less than if exposed to air. Yet it still dries rapidly to the interior during summertime weather, and the risk of problems is quite low.