To my knowledge there is nothing that is supported like that with PhysX Destruction to have the wall give way after a bit. If this is a chunk that is supported using support depth a chunk will not break free until it has received the set damage threshold to do so.
There are a couple of things that need to be noted with your settings as well.
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Form Extended Structures has a bug with it at the moment. (UE-6062) Any meshes in contact using FES will only destroy the first placed and not-necessarily the others until the original placed mesh is destroyed. This would be why in your second image you cannot break your FES.
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At the moment it is not possible as far as I’m aware for chunk that has been broken free to break apart based on it’s impact. I’ve discussed this with our Physics developer in the past, but at the moment a lot of the destructible features and bug fixes are backlogged due to other priorities.
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By having a -1 in the Default Impact Damage Depth will result in no impact damage.
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Impact Resistance is bugged in 4.7 and previous version. The fix for this will be implemented into 4.8 so that it is working correctly
I hope this helps in some capacity.
Tim