I made setup tests with
@Noztra and now I'm sure that the problem is in the tyre and not the setup, it can hardly be altered by setup. So moving away from fixed setup would be rather pointless in this regard. So I started looking at the tyre.ini and found a suspiciously low value for slip angle. Put the value to something more realistic, tweaked front and rear a bit and adjusted a heat factor to make up for the now higher side slip angle and that's it. The car feels free now without being particularly oversteery. It's not even a huge pace advantage unless you start driving more aggressive what you couldn't do before. We got used to heavy trail braking with this cars to make it turn in, need to dial it back with the tyre change.
To make it clear, it is not intended as a last minute change for Zolder. Maybe for R2 or 2nd half of the Season if the ppl that putting all the work into the league are willing and have the time to try it.