I think most of the parts you built your machine with are more of a soft spot in that device, than the belt.
The machine is not rigit enough to do high speed and deep cuts in aluminiun, so you don’t need belts for that.
Gt2 6mm is rated with 40lbs, thats 18kg. I think your printed parts will give up earlier!
It can do aluminum now. My build is 5 foot square and I’ve done these. Go slow, I think I was doing half millimeter depths, don’t remember, one of the pictures might have that info. The z touch off plate was a screw up on my part, not the machines. I think I tried to go too fast, or there was a chunk of slag in the pour so it was harder than the machine could cope with at that speed.