This not likely to be the Z axis, but rather the verticality of the router in the mount. Search for “tramming” in the forums and you’ll find many ways to correct the tilt of the router.
You can also look for “Tramming tool” which you can make or print to insert into the router collet to check the tilt.
https://forum.v1e.com/t/issue-with-spindle-tilt/16720
Mike