Brand new roof covering with insulation and sealing

Hello there!
Yesterday I had two more appointments with roofers and it seems that everyone agrees on the roof construction. So, cavity insulation between rafters (rafter width 180mm available) made of mineral fiber or rock wool and then additional insulation above the rafters of 35-60mm wood fiber. However, this only meets the minimum energy saving regulation value of 0.24 W/(m²K). This results in a cost of just under 300€/sqm, considering one new roof window and three existing ones.
If you want to take advantage of KfW funding, you probably have to achieve an insulation value of 0.14 W/(m²K), which is reached with an 80mm doubling, 260mm cavity insulation, and 100mm insulation above the rafters. The current offer is just under 400€/sqm without funding.

Just for your information first.

Hello everyone!
We can finally produce wire pins with heads now. As can be seen throughout the thread, we have already had a few appointments with roofers, all of whom would apply an external insulation above the rafters. Since we are converting the attic for residential purposes, good heat protection is definitely important.
Now we are wondering what thickness of the external rafter insulation is reasonable or sufficient. Everything from 35mm to 80mm is conceivable. Is more better? We have 180mm available between the rafters. I would be grateful for your experiences...
Thank you very much in advance!