ᐅ Question about Adding Roof Insulation Afterwards

Hello everyone,
I have a question that I haven’t found an answer to yet.
In my house, there is a stairwell with a dormer, making it very tall. Above the door to the second floor, there is a free space of 1.4m (4.6 feet) up to the ceiling. Overall, this space amounts to more than 3 cubic meters (about 106 cubic feet) of unused volume. The heat from a wood stove in the hallway below collects directly in this area, so it takes quite a while before you feel any warmth downstairs.
I would now like to reduce this space by lowering the ceiling. The current ceiling structure is as follows: tiled roof, vapor-permeable board, 180mm (7 inches) insulation between rafters, vapor barrier, battens supporting, panel ceiling. I plan to lower the ceiling by about 80cm (2.6 feet), and my question is: what should be done with the space in between?
a) leave it empty
b) fill it with insulation material
c) if using insulation, which type would be suitable, and should a vapor barrier be installed again before the insulation?
d) can the old panel ceiling remain in place, or should it be removed, since it is wood?

I would appreciate suggestions on how to proceed as well as any critical feedback.

Regards, Ralf

Hello,
if you had uploaded a photo as well, it would be easier to understand the written content...

Hello,
I don’t have a photo available at the moment. To explain, it’s simply about reducing a high ceiling by installing a lowered ceiling. My questions refer to the cavity that is created—what do people usually do with that space?

Hello,
I am leaning towards a+d! From an insulation perspective, nothing needs to be added, and a vapor retarder doesn’t make sense either... Maybe you shouldn’t build it completely airtight so that some air can circulate and the air doesn’t become stagnant there...