Are large-format tiles possibly an option? The contiguous area isn't that big, I could imagine that there are tiles in that size. And then underpin with Jackoboard or Wediplatte for the thickness.
But then that would be a concrete countertop directly and not just a filler mass
Yes, you said at the beginning that the statics would not allow a proper concrete worktop. But if the "proper concrete worktop" becomes lighter by using styrofoam, that might possibly still be an alternative to filling. That's what I meant :)