I would say if you are going big, stick with conduit. It is thin walled and rigid, low moving mass.
Are you going to be cutting 4×8 parts or just don’t want to cut down a full sheet? You can easily make a full sheet pass under it and slide the sheet down each cut. I have never really seen any giant parts cut ever.
It isn’t necessarily the weight of your tool but also the distance your spindle is offset from the gantry. The dewalt has plenty of power and it is pretty compact. If you stick a giant tool on there it increases the gantry lever arm and significantly reduces rigidity.
The laser would take at least 2 passes to cut 1/4 ply, but maybe the new higher powered one would do it faster? Not sure. 1/4 ply mills much faster than a laser cuts it, I believe that is why most only etch with them.
If you are going to use pulleys build a nice torsion box and you will be fine.