Expanded attic: What to do with existing insulation?

With 6 cm additional insulation there will be problems, aluminum is a vapor barrier.

If it is to be done properly, remove old insulation and work with at least 16-18 cm of 0.35, preferably 0.32 insulation.

For thermal protection, consider double cladding or Fermacell.

If you want to do it lean and cheap, clearly just repair and put boards on top. The aluminum foil basically acts as a vapor barrier, and if after 30 years there is no moisture in the insulation, it is obviously good enough. With the current setup (assuming the old insulation has WLG040), you get a U-value of about 0.37 W/m²K. That is not good by modern standards, but it is still very effective insulation. For comparison: an uninsulated roof would be about 4.61 W/m²K, and if you add another 10 cm of WLG032, we would be at around 0.171 W/m²K. But then you definitely need a new vapor barrier; otherwise, the new insulation will become damp. The question is also: do you have the height to add, for example, another 10 cm, or does that significantly restrict usability? The height will probably be critical, so actually only a replacement of the old insulation would be an option. But 10 cm with WLG032 is not enough either, so you have to double up. In my opinion, all this is too much effort for the slightly better insulation. If the roof is completely redone in 20 years, a proper insulation can be put on top, and that’s that.

In this situation, I would consider not aiming for a combination of internal and external roof insulation. It doesn't always have to be all at once.