Hey again,
Thanks for the quick answer, I’m a bit under deadline pressure so this is very helpful! 
If you don’t want the force of the player capsule damaging the wall you’ll want to set Impact Damage to 0.
Unfortunately, I have tried all of sorts of settings and nothing seems to work, as does an Impact Damage of 0. I can still completely demolish everything by simply touching it.
Did you set any chunks to never be destroyed?
Yes, I do want the floor bits to stay intact and have set those to Do Not Damage, although this does not seem to work for me. It gets damaged anyway.
I understand how Damage Cap works now, but won’t be needing it for now. Thanks!
The issue about the fracture piece cap was answered by Pierdek.