I think no matter which harness you go for, and however long you “try”, them in the shop by dangling off of a rig fixed into the ceiling, all waist harnesses ride-up to some degree. Shops can't simulate that aggressive upward yanking that is experienced whilst out on the water.
Some fit some people better than others of course, but unless you are blessed with a torso that is so shaped that the harness can't physically move up, I reckon it comes down to adhering to the above points- i.e learning to ride with kite lower to the water and/ or trying the looser harness approach to at least enable the harness to slide back down after it has ridden up.
Other than that, ride unhooked or switch to seat/ boardies type harness or try an impact vest