Analogy is always a good reference. Yes, you're right, plenty of aluminium framed designs without so much bracing. As I said, part of this was to provide a strong, rigid base for the bed (to avoid sag), but I've finally clicked why you used 100x50 horizontally - to provide a base for the bed down each side of the X axis (for info, my very first router was a 2nd-hand MD affair and I could measure 0.7mm deflection on rails over a 400mm span... I don't want to go back there). I've got to be careful not to go back to my first post here - I could design and build a desk-top router and simply bolt this to the existing timber bench surface which would provide some mass if only limited rigidity. For me space is the constraining factor here, more so than cost, and the existing timber bench offers me storage and a little work area.

But, I do like the portable table idea, and steel is cheap (argon, on the other hand...). I might revise/simplify the table base (which may start to look even more like AVOR with the 100x50 on the flat). There is a lot of deflection across the 1200mm span with 50x50x4 (and 50x100x4) which was the intent with the support leg to transfer some of the load into the second, lower 50x50, though I don't know if that is better implemented with a deeper beam at the bed (elsewhere on here I did ponder whether transferring a load into a lower, unsupported rail is sensible or not)... but yes, some diagonals to transfer load into the legs.

Yes, I have a metal-cutting chop saw. It was one of those machine-mart moments when you go in with £30 for a hand-held angle grinder and walk out £180 poorer with a Makita chop saw. I don't fancy balancing 6m lengths of steel box on a band-saw table :) (partly tongue in cheek - I'm hoping to source from somewhere that will cut down to a standardised "usable" length - most places do)