ᐅ Cladding collar beams in the attic / making them walkable

Hello community,
our single-family house with 1 1/2 floors will have a wooden beam ceiling.
On the underside, a counter battens system with gypsum board is planned. The attic/roof space will be insulated along the roof slopes and sealed with a vapor barrier. Currently, it is not planned to cover the roof slopes inside the attic with panels. On the upper floor, the roof slopes will be covered with double layers of gypsum board.

Since the attic is also intended to be walkable, the collar beams, which will not be insulated, need to be covered. What would be the best material to use here, possibly also to improve soundproofing/impact sound insulation? Can I install OSB boards without any problems since the vapor barrier is installed on the roof slopes, or could there still be a risk of mold issues when using OSB boards?

Thank you in advance.

Best regards,
David

Oh, one more thing: a mechanical ventilation system with heat recovery is planned for the living areas and could also be installed in the attic if needed.

Hello, is this topic too sensitive? Or did I express myself unclearly?
I made a drawing. It concerns the brown line. The question is whether I can simply screw into it, or if, for some reason, an additional vapor barrier is needed, or if I don’t need one because there is already a complete one on the outside. At least that’s how I understood it.

Regards, David