You said "when I do tufa", do you leave those pieces out over Winter?
It might help if you take a look around at nurseries or neighbors yards and see if there are stepping stones with small crevices left out over Winter. You'd definitely have to make them about 2" thick but the problem could be the veins. If water freezes in the veins it could crack the tops. My leaves aren't stepping stones just wall and garden art and I stand them up in large plastic nursery pots under my deck in the Winter. People that make Elephant ear castings will tell you that the first place they break is along the center vein due to it's depth. Don't remember what zone, but someone on this or the Hypertufa forum made square stepping stones with leaves imprinted in the tops. Maybe try a couple of each style and test them over Winter. Good luck! Billie