I have used foam core products 6mm (0.25") to 25mm (1.0") thick. The stuff I was working with has fiberglass strands mixed with closed cell foam, with a layer of chopper strand fiberglass on each side, about .5mm (0.020") thick.
strong and light. example..... 17mm thick, about 5/8" seemed to be about as strong as 5 layer plywood 13mm (0.5") thick, but was basically waterproof and weighed less than 6mm (0.25") plywood. The cost of this stuff was expensive, and I do not know about toxicity if burned. I think this get used for rebuilding boat decks amongst the more popular uses. cut and treat like plywood, wear particulate masks, I was using epoxy to glue edges together, no metal fasteners. neat stuff.... google "foam core fiberglass composite" lots and lots of stuff out there.