You steped in this one, buddy, when you hinted at something interesting to people with nothing better to do than Cletus McDingDong FEA's.
Your design isn't awful, just needs refining. But you've got about 10.2 jillion steps to go before a brake or indexing shot pin comes in to play...
As to the framing, just use square tube and weld it all up - don't bolt it. Grind the welds flush and have it stress-relieved. Then bolt on side panels and grout them in with epoxy - do the same for all the other precision surfaces. You'll basically make a stiff but out of parallel/square/whatever frame, and all the precision surfaces will be screwed to that and grouted with epoxy. You do one surface at a time and nudge it in before the epoxy sets. Once the epoxy sets you torque all the surfaces/plates down and then bolt on the moving elements.
Easy, cheap, just takes some care during assembly. And if you have access to a decent sized surface plate it becomes significantly easier - the frame Y rail surfaces are done upside down as a pair so the rails are completely parallel in Z. Same-same for the X-rails mounted on the gantry.
And if you want to dive down
another rabit hole, look in to filling the voids in the frame (between stiffeners) with epoxy/polymer granite.