Attic Expansion - Insulating the Pitched Roof from Inside - Totally Confused

A very good evening to everyone!

My girlfriend and I are currently having my grandparents' house renovated in order to move into it in the fall. It is a matter close to our hearts, and we are making every effort in all aspects to get the best out of the existing building fabric ([Baujahr 1952 Massivhaus mit Schwemmstein wurde als Haus für Flüchtlinge nach dem WWII gebaut]).

The next major step is the conversion of the attic. Since we are both more or less laypersons, I am currently quite unsure and hope to get help and assessment from the community here.

We are receiving different statements from various parties regarding the procedure and I am totally uncertain about how to proceed.

It is about the insulation of the pitched roof from the inside. A combined between- and under-rafter insulation is to be installed (is that the correct way to say it?). According to the craftsman, the following setup makes sense and will be implemented by the selected company: between-rafter insulation, then a vapor barrier, then under-rafter insulation, and on top an OSB board. Finally, drywall panels. The insulating material is Knauf WLG32 mineral wool. The vapor barrier is a product called Knauf Insulation LDS FlexPlus.

My actual question is the following. The craftsmen say that the insulation layers must be ventilated from behind. So for example, with 14 cm insulation there must be 16 cm of space. The building materials dealer, a friend who is an architect, and another craftsman in my circle of friends said that this is no longer common today and that with 14 cm insulation 14 cm of space would be sufficient.
How do you assess this? Can this be answered in general? Are there relevant arguments against the general setup? I have read about quite different possible structures here.

I do not want to presume to tell a craftsman how to do his work as a layperson, but I think my uncertainty is understandable. Also, I would simply like to understand it.
I would lose 4 cm of space on each pitched roof side. That is a lot, and if it is not necessary, I would like to avoid it.

If further information is needed for a better assessment, I will gladly provide it.

Thank you very much in advance for your support.

Especially a post-war construction year like 1952 makes it seem very advisable to illustrate the house here with plans - at least with a section, but floor plans do no harm either. The layman often has a wrong idea of the durability of the savings constructions of that time.

Thank you very much for the response. I do not understand why plans of the house help to assess the construction of the insulation. The roof construction was assessed by a structural engineer and an architect. It is only about the construction of the insulation. I have attached two photos. Maybe that helps.

A ventilation gap would be new to me. Rather, one should have as little space for air as possible in order to avoid condensation; insulation material should not be compressed, but it must fit properly. That is what I had read back then and that is how it was done for us (built in 1921, insulation between rafters and under rafters). Because of the thickness and the structure, I could check again if you want.
However, we have a different roof structure. For us, the tiles come on the outside, then the vapor barrier, then the roof truss, and there the insulation begins as mentioned above.
However, I am also a layman when it comes to insulation and the interior roof insulation was one of the things we left to professionals back then. Please consider this only as a report of personal experience!

I checked: We have 18cm Klemmfilz Integra WLG032 as insulation between the rafters, then it was doubled up with 40x60 wooden battens, Intello moisture-variable vapor barrier (sealed airtight), the undersparren Klemmfilz was also WLG032 at 30mm thickness. Then came a wooden substructure and on top of that the 1.25cm gypsum plasterboards. We did the filling and sanding.

I also read a lot last night. Whether the ventilation is necessary or not probably depends on how diffusion-open the "Unterdeckbahn" of the roof is. I have the impression that the older the house is, the more often I read that ventilation is necessary. But it is incredibly difficult to assess the quality of the statements.