Remove old insulation and replace it?

Hello everyone,
we bought a house from the early 1980s (solid construction, the roof is still in very good condition). We are currently facing the decision of whether to insulate the sloping ceilings anew. We have spoken with an energy consultant, a drywall installer specialized in insulation, and various craftsmen and friends.

The condition is, to better assess it if needed:
Double glazing (wooden windows) remains, the masonry has an air layer and 40mm insulation, two roof slope windows will be replaced.
The roof is currently insulated with 140mm glass wool (one side aluminum foil laminated) between the rafters, in front of the roof tiles there is a kind of "grid foil," inside the plasterboard is attached to the sub-rafters.
The current insulation goes from the roof gable all the way down and the floor ceiling of the attic (gable attic) is also insulated. The knee wall (Kniestock) of the rooms is solidly built at about 1.2m, behind it there is still insulation.

To receive funding, the effort would be too great. The plan would be to bring the existing rafters to 200mm and then insulate with glass wool wlg 032. The drywall installer would demolish the knee walls and rebuild them with drywall. He said it would be sufficient if we only do this for 2 children’s rooms and the bathroom. The bedroom and staircase would remain as they are now.

Does such a partial insulation really bring that much?
Is it even sensible (thermal bridges, etc.) not to insulate everything anew (bedroom, staircase)?
If the roof needed to be replaced in about 15 years, could it be insulated better then?

We are really torn because it was not planned and we do not know how much the effort "brings." Costs for the 3 rooms will be just under 10,000 euros (plus electrical work and removal and disposal of old insulation etc.)

What to do? We are really at a loss....

It would be really great if someone could give an expert opinion on this or if someone has already done something similar. :)

We are currently facing the decision of whether we should insulate the sloping roof areas anew. We have spoken with an energy consultant, a drywall installer who specializes in insulation, and various craftsmen and friends.

There were experts on site who looked at the house. You formed your opinion accordingly. Do it as planned, none of us know your house. What has the energy demand of the house been so far? Whether it will ever pay off, only the crystal ball knows.

There were experts on site who looked at the house. You formed your opinion afterwards. Do it as planned, none of us know your house. How high has the energy demand of the house been so far? Whether it will ever pay off, the crystal ball knows

Thank you for the answer! Since we do not live in the house yet, I cannot say how it is in summer and winter and whether the insulation is currently sufficient. According to the energy certificate, the energy consumption is about 150 kWh.

Do you then save about 20 percent energy?

Demand or consumption certificate? Who lived there before? Obtain old bills, how warm was the heating, all rooms? Then it might be possible to estimate that roughly (!).

Demand or consumption certificate? Who lived in it before? Get old bills to see how much heating was used, all rooms? Then you might (!) be able to roughly estimate it.

My energy certificate says the building has an energy consumption of 150 kWh.
Old tenants lived there, heated more than well. Statement from the daughter.

Is the surcharge for glass wool WLG 035 to 032 worth it?