Is that a single rock on the left or a stack of lots of pieces?
It's a stack. I really worked with the little pieces I had and found the best (imho) combination.
IMO your main problem is the rocks are not big enough nor high enough. You said you used the smaller ones. I'd be trying some larger ones as well.
Generally you should be trying to get your highest point somewhere around 2/3 of the way up the tank. At the moment it looks as though you aren't even getting half way. One reason is when you plant it the rocks will soon be covered in plant and you won't see them any more. I was talking to a guy who does aquascaping for a living and he told me he always puts his rocks on other rocks or such so they are higher than the substrate. This way when you grow a carpet you don't loose the rock formation you spent ages getting to look good. It's easier said than done though, rocks are easier to put in the substrate than on other rocks.
I see your point here! I've got about 70 lbs more of the slate, so I think I will find some slightly larger "base" pieces and put them under the one on the left to get it up to the 2/3 mark.
In Iwagumi you have one main rock and all the other rocks have to be subservient to it. As in the others have to compliment it. The main rock should be bigger than the rest and placed first, then work from there.
I think once I address this issue, all of the other points will have returned to square one, so we'll see where it goes from there. The one on the right will probably have to be completely re-evaluated.