I would suggest you need to dial in the steps per millimeter for greater accuracy and tighten up the belts to the same tension. My x carve has an aftermarket z axis and 9mm belts, risers, stiffeners, and extra extrusion for rigidity. It’s a pretty capable machine and can get pretty good accuracy. Good means it isn’t perfect though like a 40000 dollar milling machine might be.
Calibrating my X-carve 1000 - X-Carve - Inventables Community Forum