The high force you measure moving the surface D in the Z direction is because that BC keeps the surface flat, but in reality, that surface rotates as it moves to open the contact.
Instead of using a Displacement on the surface D, use a Remote Displacement, Behavior = Deformable and enter the Z displacement leaving all others Free. Now the surface can rotate as it deforms. You will find the Reaction force from the Remote Displacement is lower.
Once you have a Remote Displacement (or plain Displacement) as a BC replacing a Force, you no longer need the other side of the contact. Suppress that part, the solution will be faster.
You should also try creating elements half the size, or smaller to get 2 elements through the thickness. This may lower the force further.