I'm just going to say what popped into my head when I first looked at your scape, that large stone on the left is just in the way it seems. May I suggest taking it out, maybe breaking it up into a few smaller pieces and then adding those back in its place? I've never done an iwagumi but I've seen a lot and my eye typically prefers an odd number of stones and smaller stones (about half the size) of one larger stone all "pointing" the same direction. This is just my opinion, but I think you picked out a great stone for hardscape. I keep seeing these spec tanks and its making me want one more and more. Looking forward to seeing what your end result is!