Insulation of interior walls, but how??

Hello, I urgently need tips regarding my roof conversion. The roof itself has now been completely insulated with above-rafter insulation (diffusion-open system/vapor barrier). The house is from 1972 (Schwörerhaus) timber frame construction with 140mm exterior walls. -First of all, it’s about the interior insulation of the exterior walls, I would like to clad them with, for example, Sonorock Plus and studwork, the question is whether I should then cover it with a vapor barrier or a vapor retarder before installing drywall? -For the interior walls, I would like to build a wooden frame construction with OSB and drywall on both sides and insulate the inside with, for example, Flexirock; question: do I also need to install a vapor barrier or vapor retarder here? -In the bathroom area a vapor barrier??

For interior walls, you do not need a vapor barrier. For the bathroom, the green Rigips boards.

You should stay away from the exterior walls, as you obviously do not know what you are doing/need to do. You should urgently seek professional expertise for both planning and execution.

An architect once wrote that above all it must be ensured that the interior insulation cannot be bypassed. So the substrate must be either completely smooth or the insulation material must be flexible. In addition, there must be no cracks or joints where air can enter the insulation.
Therefore, for the interior insulation made of wooden battens and mineral wool, we glued a vapor retarder (climate membrane) - not stapled, as that would create a thousand tiny holes again. There are special cartridge adhesives for this. The joints and connections, e.g., at the window, with special tape. Especially for the window connections, this is important in my opinion, because otherwise you simply cannot achieve airtightness.
It is actually redundant, since we have a curtain wall and any condensation water can be ventilated outside anyway.

But it is also said that an OSB cladding is itself vapor-tight. At least if the joints are really sealed tightly.

For interior walls you do not need a vapor barrier, since interior walls cannot cool down enough for condensation to occur there.