Insulation of the attic as living space

Since we are expecting a baby soon, we have made the plan to convert the attic into living space to get an additional room ... we have already accomplished a lot in terms of craftsmanship but the exact plan for implementing our project is still missing. The roof is covered with slate on a wooden sheathing (one side new with a breathable membrane, the other side older with tar paper) and the rafters are 12 cm wide. Now we want to insulate, comply with the minimum insulation thickness, but also not build up too much, otherwise the room would become too small. What would you suggest? In the first picture is my idea, what do you think about it? And the other pictures show the actual condition...

Firstly. Toom Baumarkt is currently running a promotion on isover insulation wool at a special price. If someone is nearby, take advantage of it. The better 120er 032 costs about €8 per sqm.

Then the rafter thickness is sufficient. On top, a breathable climate membrane that allows moisture to escape again. We used siga crall as tape, it sticks like hell. As a foil adhesive on the wood, an MS polymer mixture in cartridges, no name, store brand from the building materials trade. But it sticks. To stabilize the foil, we used cheap formwork boards. See picture.

At least the roof side with the roofing felt is too tight to insulate as usual. You need to create a ventilation layer between the boarding and the insulation so that any moisture that gets in can dry out again. This requires, on the one hand, an air gap below the boarding (i.e., do not fully insulate the rafters) and, on the other hand, appropriate ventilation openings at the eaves and ridge to ensure airflow there.

The use of a climate membrane is almost mandatory in this case, and it is also advisable to use sorptive insulation materials such as wood fiber or cellulose. Glass/mineral wool is not capable of absorbing significant moisture and should therefore preferably not be used in this case.

Many thanks in advance for the quick replies! The first option obviously sounds simple. Is there another image without the insulation wool and will it then be covered with plasterboard or something similar, or will it stay like that? I think implementing the ventilation will be difficult because the roof made of slate definitely cannot have a ventilation opening at the top, right!?

If in doubt, ask a carpenter. No drywall for me. It is an attic that will remain uninhabited. So why? Here once again floor before insulation.

The first variant obviously sounds simple

I didn't want to present you with a particularly elaborate variant to show off or annoy you. Unfortunately, due to the roofing felt, you have an external vapor barrier. That must be removed or appropriately taken into account. Search engine: self-composting (flat) roof