Roofing insulation nightmare
I have nearly the same situation, although we have 1" of rigid foam insulation over the T&amp;G. We could have added insulated roof panels when the roofing was replaced, but it was very expensive. The materials aren't cheap, and further it requires replacing the fascia to accommodate the increased thickness.

It's not an easy project. Imagine replacing the roof. Now imagine something much harder. I wouldn't attempt it without help, and I'm a lot younger than 72.

It's possible you could sneak some rigid insulation and a layer of decking in when the roof is replaced, and cover it with extra wide drip edge to avoid replacing the fascia. It's not going to be great insulation, but compared to none at all it's a significant improvement. The seams between panels should be taped to provide a vapor barrier. If the warm, humid interior air can't leak through to the cold exterior and condense, you don't need ventilation.

https://buildingscience.com/documents/digests/bsd-149-unvented-roof-assemblies-for-all-climates

If you are looking to save some money on heating and cooling costs, I'm almost certain there are more cost effective things you could do. In my case, it was more cost effective to take the money that would have been spent to install insulation on the cathedral ceiling, and use it instead on a more efficient HVAC system.

Better still, a house built in the 70s is probably full of air leaks. This is something you can fix yourself, with very little money, without the hazard of climbing on a roof. It's also probably the only job where a can of Great Stuff is legitimately a good idea.