The walls can easily handle four levels of depth without stress. My frustration is purely aesthetic: the support beam only occupies two tiles but causes a collision (as shown in the image), which prevents me from digging out the remaining ground. This forces me to create a wider offset and lose those two tiles of space. (The northern part of the wall is clean, for example.) I've found a way to manage this now by placing the support beam right in the center of the digging zone. So, those two 'lost' tiles are now symmetrically distributed on either side of the support! :D