I'd shim under the top piece of plywood (NOT directly under the tank) to bring the middle part up to support the tank before using foam - one shim every 3"-4" or so.
After the bottom of my 6-footer cracked even with foam underneath it I spoke with a number of woodworkers and a structural engineering friend (albeit in less than sober state
) and base this on their input.
On a GLASS tank you don't need the foam their as long as the perimeter is supported.
Insulation foam won't "squish" down much all at if that's what you're counting on - certainly not 2mm. You'll still probably end up with about a 2mm gap between the foam and the stand if you don't fix the stand itself.