The valance is a piece of 6061-t6 aluminum below the tail gate that overlaps the corner armor right up to the radius. It is bolted to the torque box with nutserts where it can't be through bolted. This unifies the entire rear of the body with bolts instead of just a bunch of small spot welds. It ties the left and rear corners together. Your new cage extensions get bolted through the corner armor that is now firmly connected to the entire rear of the body.
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If you have an existing valance under the gate, use 1/4" thick material to cover everything. If you don't have an existing valance under the gate, use 1/2" thick material and rabbet out the thickness of your corner armor at the ends. A router or a table saw can cut the aluminum.
Suddenly now you don't want to overthink things? You added cage supports. At least try to make them do something.
I get SHIT from people all the time for trying to OVERTHINK things. And I already do think the added cage support is doing something. Might not be as much as how you are describing it but it does spread the weight out from just the tub upper mount point.
These are the only valances I've ever seen. I've never seen one that does what you're describing.
https://genright.com/products/aluminum-filler-plate-for-rear-body-panel.html
And yes there are MANY times I can't visualize things until I see a picture of it or have it broken down and explained to me but I get told to just figure it out for myself. So I stopped asking. Not all of us can just look at things and immediately figure out a solution to it.