Place the tongue and groove boards between the rafters. Without underlayment/insulation

Does the following setup, as shown in the picture, perhaps also make sense?

That’s beyond me.

I would say: give it a try. That means: start as you initially intended – that is basically like the first tongue-and-groove layer of this setup. Then you first see how far that gets you, and then we can discuss retrofitting again. Probably about a year after the first measure, I think.