I make my living with welding and have big strong (200ton tested) jigs I could use and know how to hold weld distortion to minimum :)
But still I like your idea for the bed more. The massive amount of steel I designed there was in a hope that I could maybe use 15mm ali bed and stop excessive movements with the frame underneath.

So with the Y you basically suggest a structure shaped like a PFC?

Z axis plates? You mean the plate where the spindle and Z axis rails mount to? It is intended to be 20mm. At least in my head, have to check the drawing tho :)