Yep, looks like it'll save me about $700 in blocks using 6". Plus the savings on concrete due to smaller cores. I know what you mean by bending the rebar into Z shapes to get one continuous bar in there, and I'll do that.6" cmu's would be nice to work with. I'm a little surprised they are cheaper.
I had 2 step downs. I suggest you "overlap" the footer by about 12-18 inches where you step down and bend some rebar to make that extra strong. Overkill, I am sure, but pretty inexpensive.
I wonder how I should handle the floor. I don't care for the traditional "hopper" design for a deep end. I like the idea of straight walls going down full depth. And only a slope from shallow to deep, straight across the floor. Simpler to build, and measure for a liner.