By benching do you mean the incompatibly line at the graft union? What limiting factor does benching have on the scion? (With apples it either takes or it doesn't pretty much, so just trying to clarify.)
With FD I was under the impression that while it grows slowing, it also induces precociousness, no?
Why is hard tap water a killer for FD? I have hard well water which then goes to RO.
FD starts to have sprouts which will take over the graft if not maintained. Precociousness is a long story. It doesn't really matter for your purpose.
FD likes slightly acidic soil, Iron will not be available at higher pH, sensitive to calcareous soils and a high salts content kills the tree. The main the reason is slow growing that it's not used on commercial applications.
I also use it for decorative trees to have few fruits on the tree in a small pot for citrus not so important.
The thing is you plan to plant them in large containers and you expect to eat from them I suppose.
Moving a FD recently grafted to a 15 gallon container will probably take you 10 years or even more whereas with another rootstock it could be in 3 years, but then there will be the job of trimming and root pruning.