We call it crusher dust out in the West here. That sounds like a ton of work! My tub sits on barkman patio block (roman pavers) and it is just fine. It has crusher dust underneath it of course but with all the wet heavy snow and floods you guys get sometimes, if that crusher dust isn't compacted absolutely perfectly, you are going to have issues. It is supposed to be as hard as concrete when it is done but man, you have to compact it perfectly.
If you want to put in crusher dust, compact it, and lay some patio blocks like I have, I think that would be best. Use polymeric sand so the joints are hard like concrete and you shouldn't have issues. We have pretty bad frost heave where I am (8' frost line) and if you ever need to relevel your tub you can just adjust your blocks if you need. Or concrete, but if your worried about heaving, that probably isn't best.
Forget the foam and plywood imo.
