That is a lot of stones to be loading onto those stools. While switching to Hickory should add at least 30% greater strength, Im not sure I would want to use a round leg instead of rectangular in section, at least at the pivot point. The wider face to face contact area could really take some of the stress off any torsion from the activity above, which would all be between the hardware inside the round hole for the round section.
Shortly after I finished the Sunroom on the back of our cabin, I ran across some neat folding moon chairs and a rocker that I really liked at a outdoor store, so I bought two of the first and one of the rockers. When I bought them, the label said maximum weight limit of something like 225#, which I had forgotten about since none of us were that heavy. Then we got some new neighbors, father and son, both of which were over 300#. The first time they visited, dad broke the rocker by trying to spin around without getting up, and not long after that, the son plopped down into one of the moon chairs and ripped a seam.
The reason for sharing that, is that once it's made and away, you have no control over how many stones are dropped onto it.
If you went with a rectangular section, you. could lighten it by tapering towards the ends, or routing out to make essentially a I-beam, so you had more fibers spread apart, but light in the middle, but that might not look like you were wanting.