Hi triumph406, good to see you're still active here, I for one, value your input.
As to the question of boltzzz design, I personally don't see the need for pins, as I mentioned, not trying to open that can of worms again, but I did think to suggest one thing.
Square aluminum tube (not T-slot!) perhaps 3/8" or 1/2" thick, could be filled with epoxy granite or high density polyurethane, could even press in plywood ribs, and a bolt together design without welding.
Aluminum generally comes flat enough for our purposes. I suggest taking a long straight edge to the metal merchant to check out the flatness before buying.
This is different from the original design as it doesn't involve trying to bolt together all different shapes to make a gantry, but actually using aluminum tube instead of steel because it is easier to work with and generally comes flat.
Steel is stiffer than aluminum, but aluminum weighs less, so you go with a thicker wall and consider filling it, if the pieces come flat enough, perhaps being easier to work with might justify the added cost in this instance.
That's really all I have. Good luck, and keep us posted boltzzz.