Reconstruction of insulation in the old building

Hello to all building comrades. I myself have already built and am therefore familiar with the concept of insulation from the living space to the attic. (Insulation wool between rafters->vapor barrier nailed from below-cross battens->drywall ceiling) So, now an acquaintance has bought an approximately 100-year-old house. Structure of the ceiling from bottom to top (cold roof): plaster-reed composite (reed nailed), planks/boards perpendicular to rafters-> steering layer planks/boards between rafters on lower boards (each 20mm thick)-> space filled with slag/straw mix. My plan was to knock off the plaster/straw ceiling, empty the spaces, remove the layer of boards that lie parallel to the rafters and replace everything with insulation wool. Then to attach a distance batten to the lower boards, install a vapor barrier and then rehang the ceiling with drywall. However, there were repeated discussions about this. Leave the plaster-reed ceiling on, only remove the slag material and replace it with wool, and then install drywall ceiling. The goal in my eyes is to relieve the roof and free it from old insulation methods. I personally find the variant with the vapor barrier to be the most energy efficient. Now unfortunately I do not know whether I also have to remove the bottom layer of boards in order to correctly install the vapor barrier-> wool variant, or whether this layer of boards can remain. (They are laid edge to edge)... Or should the old ceiling even remain and only the slag part be replaced with wool where the vapor barrier is completely omitted? I hope I was able to explain it somewhat pictorially. Do you have any experience in this regard or are there any direct specifications? I would be very happy about constructive help and advice. Best regards

PS: I just saw that I posted it in the wrong section, but somehow I am unable to delete the post.