Sloping roofs, double-layer OSB and gypsum board for thermal insulation

OSB is indeed a vapor barrier. However, I see no problem with it. OSB has mass and mass helps with heat protection. I have not yet installed wood fiberboard. But it could be a possible solution

The problem lies in the glue of the board. If there is dew water (condensation), it inevitably leads to mold. But why force wood boards. GK is good after all!

That's why the underlay membrane is diffusion-open

Yes, that is the case.

But why force wood panels. Gypsum board is good!

We missed the rafter insulation but I still want to do something against the heat. 240mm wood fiber insulation boards cost around 30 EUR/m2, three times as much as glass wool. An additional layer of OSB or gypsum board is a comparatively small extra cost.