What’s the goal with the aluminium extrusion bed? Is that machine rigidity or just ease of workholding?
For the Y plates, if you go to 1/2" plate there will be clearance issues unless you shorten the X beam, might be easier to order new steel plates from somebody like send cut send.
In terms of deflection in the Y plates I’d suggest the plates (assuming you’re using equivalent thickness steel) won’t be the major issue, the plastic V wheels will dominate your deflections on this machine. I built up a model of this here
Extending the plates up above the Y rails and wheels may apply more leverage on the lower set of wheels and increase deflection. Mostly the plates are rigid because of the X beam bolted to them, a short length of angle or extrusion on the outer face of the Y plate to keep it rigid across the gap between X beam and top V wheel may help, you could add that later on if needed.