(Ffm like Frankfurt am Main? I am from Neu-Isenburg) I understand your uncertainty and know it. In the end, I am also just an amateur and have read into the topic. One of my predecessors wrote: mass is everything for heat protection and soundproofing, and he is right. And mass does not mean a disadvantage for cold protection. You just have to know how much.
Regarding the options: Insulation under the rafters is not sensible in your case, as you lose living space and you will open the roof completely for work anyway. However, instead of drywall, you can also use wood fiber or clay construction boards as wall coverings. It is a matter of money. It additionally contributes to heat and sound insulation. A renovation company I inquired suggested exactly that and referred to a list of references in the airport area...
Insulation between the rafters is sensible and reduces the build-up height of the insulation above the rafters. It is only more labor-intensive because the insulation material must be installed in two steps. First a layer between the rafters and then a layer on the rafters. With insulation only between the rafters, you will not reach the thermal transmittance value (=U-value) required by the energy saving ordinance. I think it is 0.14 for the roof in renovation measures. Furthermore, some sort of sheathing must be attached from the inside to the rafters as support for the insulation material and the insulation mats have to be individually cut to fit the respective rafter spacing. That means labor time and waste. In my house from the 50s almost every rafter spacing is different.
Insulation above the rafters is therefore good because it involves only one step: laying thick boards on the rafters or on sheathing attached to them. Roofers like to do that. The second reason: the roof is completely enclosed by a continuous insulation layer.
In new buildings, usually a combination of insulation between and above the rafters is done so that the roof structure is not too bulky. An additional 25 cm on an existing roof already changes the impression of the house quite a lot.
You could also do this combination. You can do the insulation between the rafters and the drywall/wall covering yourself. However, the position of the vapor barrier in the entire structure must be considered... and you should not make compromises there. An interior vapor barrier is sometimes difficult to install because stud frameworks or purlins get in the way. I have often read that especially here people are called on to do it themselves because the homeowner will proceed more carefully than the craftsman... Unfortunately, that corresponds with my experience. Those who build cheaply, build twice! To check, a blower door test can be done if desired.
I recommend playing through the variants in a U-value calculator. All building materials are stored there. In case of complete uncertainty, I would ask an energy consultant to design a roof structure for you that then a roofer should implement.
Alternatively: just call a field representative of a manufacturer of wood fiber insulation materials and get advice. I did that completely non-binding as well. Whether you then use one manufacturer or another is usually decided by the roofer. The insulation materials, especially PUR, mostly come from the same factories anyway and just get different brand names on them...
So get on the phone, read on the internet (Steico and Gutex have very good brochures online for your purpose), calculate with the U-value calculator and, if in doubt, contact an energy consultant or architect and tell them exactly what you want!