Summer heat protection and winter insulation - experiences?

Roof insulation with wood fiber boards. That would still bring some benefit. If too expensive, insulation on the room side under the rafters is the second-best solution.

Our attic is always nicely cool. Rooms with windows quickly become warm when exposed to sunlight. Better invest in automatic [Raffstores].

We have 240mm 035 ZwSp and another 60 mm 035 Untersparrendämmung. The USp mainly serves to blur the thermal bridges from the rafters. Overall, it is probably almost excessive.
Even at -12°, you basically didn't have to heat on the upper floor.
Now with the first temperatures over 10°, since the sun protection for the roof windows is still missing, you almost get roasted, the heat no longer escapes on its own.
Therefore, my advice is to leave it as it is and invest more in sun protection.

Ok, great!

Then I prefer to invest in sun protection.

Thank you very much

I built OSB as a vapor barrier without foil. In your place, before you screw on the drywall on the inside, I would first screw on another layer of OSB... it stabilizes the roof structure and adds extra mass.

Tell me, what do you think about insulating the top floor ceiling above the mentioned structure with cavity insulation made of mineral wool WLG 035 of the collar beam ceiling with an additional layer of Steicotop wood fiber on top.

Is this possible without any problems? Will there be issues with moisture in the mineral wool or similar? So, is the structure sensible, feasible, and compatible?

The whole thing would have the advantage that the attic would at least be walkable for a short time.