Just throwing my two cents here, 1 if at all possible, go deeper, My pond is that depth way down here in Louisiana and I have problems watching close during really cold snaps but even worse the high temps of summer, 30" should be your shoallow end to eliminate most of your pedators, I've seen crains and coons stand in 24" of water! second, once you have your dry stacking and you think you need more support, Waterfall foam does great, just shoot a dab between the rocks and it will fill in gaps but still give some support to keep rock slides down, Need to let it dry for 24hrs but it's 100% safe and doesn't need anything done to it to keep water PH stable like motar. The only time I would use cement would be for wall construction then I would seal it.