FromThe cross-section of the following three sections legs will be doing most of the supporting for the same thickness andgranite weigh in this case. Even the same heightmaximum of weight 238kg, is not going to be too taxing on the most efficientsupporting frame. (making the most useFor example if you had vertical legs a flat bar of the material ) willalmost any thickness more that 3 mm would be the one in the middleOK).
Since you have this small inward angle, there will be some small danger for buckling horizontally.
However since you are using already a 100x100x4 mm 316L Stainless SteelFor simplicity, I would go for the same,to keep matters simplea flat bar with width of 100mm and, thickness between 10 to have a more uniform aesthetic in15[mm] (that may also be overengineered) but you will be on the structuresafe side.