Question about retrofitting roof insulation

Hello everyone,
there is a topic that concerns me, to which I have not yet found an answer.
In my house, I have a stairwell that is very tall due to a dormer. That means above the door to the second floor there is a free space to the ceiling of 1.4m. Over the entire area, this results in a volume of more than 3 cubic meters of space that is unused. The heat from a wood stove in the hallway below accumulates directly in this space and it takes correspondingly long until you feel some of the warmth downstairs.
I now want to reduce this space by lowering the ceiling. The construction of the current ceiling is as follows: tiled roof, vapor-permeable panel, 180mm insulation between rafters, vapor barrier, batten substructure, panel ceiling. I want to lower the new ceiling by about 80cm and the question arises: What do you do with the intermediate space?
a.) simply leave it empty
b.) stuff it full with insulation material
c.) if insulation material, with which one and should you install a vapor barrier again before the insulation
d.) can the old panel ceiling remain or should it better be removed because it is wood
I would appreciate suggestions for the implementation as well as criticism.
Best regards Ralf

morning,
if you had posted a photo now, it would be easier to understand what was written...

Hello, I don't have a photo at hand right now. To explain, it's simply about a high ceiling that I want to reduce - to lower the ceiling. The questions relate to the cavity that is created, what do you do with it.

Hi,
I tend to go for a+d! There’s no need for insulation and a vapor barrier doesn’t make sense either... Maybe you shouldn’t build it too airtight so that some air can circulate and the air there doesn’t become stagnant...