You can get that entire reduction in 1 stage.
Lighter, simpler, more efficient.
12:72 will work with a 4" wheel, and actually gets more reduction than you have right now. (12:69 would be right about where you are currently.)
I always use a 12t or smaller on my CIMs because it allows us to put the gear on the CIM before installing it through the CIM 0.75" pilot hole.
Instead of spending money on COTS gears, you could go find a local shop who can make you some custom aluminum 72-toothers for cheap (or free). You're not going to need a whole lot of gear, should be nice and light.
-John