I'd check out Gilly's. He thinks, and sometimes outside of the box. Uses good judgement too, imo. I throw a lot of likes his way.
You can get contruction foam 3-4 inches thick. You aren't breaking it easy or getting much lighter. Im thinking metal rod "rebar" embedded from all sides might stop that. (grind the tip into a drill bit shape, drive the last inches, and plug the hole with adheasive. I don't think you'd need bit flutes.
Another member did one big sheet for a table top or wall leaning when unused, another used two sheets for a Christmas floor layout.
The green I used dented sightly under my weight, but some 4" plywood squares & my knees and hands had "snowshoes" to rest on (185-210lbs). Later, bigger ones (about 6-8"") stradled two rail lines when I threw them down. Kept those under the transformer.
I kinda like fiberglass idea if you are doing raised terrain/mountain/etc. Check with a few boat shops. There is a cheaper alternative to true carbon fiber(costly). It's stronger than regular fiberglass, and more cost effective than true carbon fiber. It looks like carbon fiber too. All use the same fiberglass resins. Boat shops usually carry the best fiberglass supplies (get ready for some possible sticker shock trains aren't expensive, boats are. I paid $35 a quart for my choice wood varnish over a decade ago. Fiberglass was cheaper though ) The stuff sets fast, there is heat. Too much to hold a containers worth setting up with bare hands for most folk. (ok while in the working stage, hottest after work time, as it really hardens.) Fun to lay, a bit tough to sand. I'd rather be low and skim coat up than sand down far.
Another l thought for terrain is expanding foam ("great stuff foam", but you can opt for a two part mix that is basically the same stuff, but you pour it where you want it. (Boat/automotive/paint supply house) These foams are great to carve on. (a local guy does fake hollow tree "stumps" 4-6ft tall for garden decoration all in foam. Looks real in person, appears heavy, feels like balsa )