Mike I am not trying to find fault with what you are doing but I was thinking about your design and I see a problem with it; you rely on your floor being perfectly flat and your caster being exactly the same ad to position your bench always exactly at the same location, to keep the in-feed and out-feed tables in line to obtain a straight surface when planing.
In case your floor is not perfectly flat or your casters not of the exact same diameter you are going to have issues.
If I was going to build such a bench, I would build the top part as a torsion box to keep it as flat as possible.
It might still be time for you to add a torsion box on the top of your bench.