Answer:
The easiest way to manipulate the stiffness is to use a new material where the E-modulus and the shear modulus are manually reduced and assigned to the corresponding walls only. As an alternative, you can use line hinges with translational springs at the connection to the floor.
Otherwise, there is the option to use a scaling factor for E and G. However, it is also necessary to create a new material for this. However, this is somewhat prone to error because this stiffness reduction has to be assigned to the desired load cases and load combinations. Thus, it is not applied automatically to all LCs and LCs, but only to the assigned ones - unless you click the "Apply Settings… for All LCs" button.
