Advice to Simplify Foamless R49 Roof Assembly

We are in the planning/engineering process of a heated barn/shop with second level living space in climate zone 5. We are using open span, Glu-Lam gambrel rafters, 12’ on center, sitting on top of Perfect Block ICCF walls. Between rafters will be 2×10 I-Joist purlins with dense packed cellulose. We would like to avoid all spray foam, and avoid any external foam due to high wildfire risk. In order to reach an R-value of 49 (or better), and minimize thermal bridging through purlins, and overhangs, we are going to need some external roof insulation. The plan is to frame a 2×6 insulated over-roof 24” OC with (preferably mineral wool) batt insulation in the cavities. The 2x6s would run vertical, from eave to ridge, and would also be cantilevered on the gambrel ends to form our over hangs. We would use 3.5” batts to leave a 2” vent space under the roof sheathing.
MAIN QUESTION…
Am I over-complicating this in an expensive and unnecessary way?
Is the thermal bridging as detrimental as the building scientists suggest?
Due to snow load (70 psf) this cathedral roof needs to be vented regardless. Would we be better off with a deeper purlin cavity and some strapping to create vent space? With that approach we would have to design some sort of externally supported overhangs, which is not impossible but may be costly.
What could we do to simplify things with minimal loss in performance?
I will attach some photos to hopefully better explain our assembly. This is an off grid property, but we certainly don’t have an unlimited budget, so a balance of efficiency and cost is what we’re working towards. Thanks in advance for any criticism, feedback, advice, etc!