I can’t comment on DOM, I have only tested stainless and conduit. Be careful, Stainless is nice and rigid and won’t rust and pit, most steel is not and has lots of flex.
Weight is a moving mass issue, belts are not going to stretch before you break plastic. The heavier it is the slower your accelerations, and cnc’s are basically all acceleration plus and minus. It has a huge effect on cut time. If you go too big it will also sag under its own weight.
What will the primary use be for your cnc?