fixing a 1975 cathedral ceiling – GreenBuildingAdvisor

I have a 1975 home in zone 4C that has 1000sf of cathedral ceiling with 2×8 framing with insulation more of less filling the cavity.  the attic space around these cathedral areas (~800SF) is under-vented.  There are signs of mold on the underside of the osb sheathing in the attic areas and the roofer, during the bid process, found at least three sheets of sheathing that will need to be replaced due to softness and who knows what they will find when the roof comes off.

I’m wondering about the best approach to fixing the roof ventilation. The cheapest would be the pull all the sheathing above the cathedral areas and reinsulate with r23 batts and being sure there is a vent space above.  That combined with adding a peak vent would seem to do the trick though might actually decrease the not great thermal performance of the roof.  I’ve also thought about rigid over the top of the roof combined with insulating the gable ends and sealing all the vents.  I’m looking for any wisdom on the best approach here.  Thanks!