Floor between roof ridge and attic insulated, roof not

Good day dear forum,

we have bought a house from the year 1966 (solid construction). Unfortunately, the roof is not insulated. The windows are all 4 years old. The house has a roof length of 13.75m and is 7.385m wide. The tiles are only 15 years old and still look good.
In the attic is the "stage". The converted attic with two dormers contains the bedrooms and a bathroom.
So far, only the floor between the attic and the top floor is insulated.

Now the question arises. Is it worth insulating here, and if so, from the inside (between the rafters) or from the outside?
There is also no foil installed in the roof, and the dormers are not insulated either. We have a budget of about 20,000 €.

Or should the money rather be invested in a photovoltaic system including a battery? The house is heated with night storage heaters and a large tiled stove.

Attached are a few pictures of the roof.

I am looking forward to your opinions & tips.

Insulating a roof without an underlay membrane is always problematic because the insulation is not protected against penetrating water. One could try to staple an [USB] in strips between the rafters or use insulating wool with fleece lamination. In any case, leave a few centimeters of space between the roof tiles and the insulation to allow air circulation. From the inside, use a vapor barrier to avoid exposing the construction to even more moisture.

Insulation is worthwhile in any case because it is comparatively inexpensive and can even be done as a DIY project. It will probably also be sufficient for the photovoltaic system in the end. However, if you have night storage heaters, I would forego the additional storage.

First, it would be easier to insulate the top ceiling if the attic is otherwise empty and unused or only storing tile remnants or similar. However, if you also want to insulate the roof, use one-sided laminated glass wool as a compression fleece, no vapor barrier, but a climate membrane, that is a composite foil that closes in the cold but opens slightly in the heat, i.e. in summer, so moisture can escape from the glass wool again. These foils are a bit more expensive, but whether you install 970 or 1089 up there is ultimately irrelevant. What is important is a completely airtight bonding of the foil. Use foil adhesive for the connections to walls, ceilings, and floors, and seal the overlaps with Sigakrall, which is a devilishly well-adhesive tape. To support the foil sheets, use boards that you screw onto the rafters and which fix and support the foil. All this can be done by two people over a weekend if the material is on-site.

To prevent misunderstandings: In my post, I refer to insulating the roof, or the dormers in the area of the inhabited attic, not in the area of the stage. As long as it is not heated there, the existing insulation of the ceiling is sufficient... but also check it once for the presence of a vapor barrier.

Thank you very much for your contribution! Would you then insulate the roof from the inside or the outside? What thickness would you recommend?