How do I get the player to correctly rotate toward the mouse?

I have this script called "PlayerController" attached to a cube(the player):

 using UnityEngine;
 using System.Collections;
 
  [RequireComponent (typeof(CharacterController))]
 public class PlayerController : MonoBehaviour {
 
     public float rotationSpeed = 450;
 
     private Quaternion targetRotation;
 
     private CharacterController controller;
     private Camera cam;
 
     void Start(){
         cam = Camera.main;
         controller = GetComponent<CharacterController>();
     }
         
     void Update() {
         Vector3 mousePos = Input.mousePosition;
         mousePos = cam.ScreenToWorldPoint(new Vector3(mousePos.x,mousePos.y,cam.transform.position.y - transform.position.y));
         targetRotation = Quaternion.LookRotation(mousePos - new Vector3(transform.position.x, 0, transform.position.z));
         transform.eulerAngles = Vector3.up * Mathf.MoveTowardsAngle(transform.eulerAngles.y, targetRotation.eulerAngles.y, rotationSpeed * Time.deltaTime);
 
         Vector3 input = new Vector3(Input.GetAxisRaw("Horizontal"),0,Input.GetAxisRaw("Vertical"));
         Vector3 motion = input;
         motion *= (Mathf.Abs(input.x) == 1 && Mathf.Abs(input.z) == 1)?.7f:1;
         motion += Vector3.up * -8;
 
         controller.Move(motion * Time.deltaTime);
     }
 }

And I have this script called "GameCamera" attached to the main camera:

 using UnityEngine;
 using System.Collections;
 
 public class GameCamera : MonoBehaviour {
 
     private Vector3 cameraTarget;
 
     private Transform target;
 
     // Use this for initialization
     void Start () {
         target = GameObject.FindGameObjectWithTag("Player").transform;
     }
     
     // Update is called once per frame
     void Update () {
         cameraTarget = new Vector3(target.position.x,transform.position.y,target.position.z);
         transform.position = Vector3.Lerp (transform.position, cameraTarget, Time.deltaTime * 8);
     }
 }

The player rotates perfectly when the camera is at 90 degrees in a top-down view, but I want to offset the camera to be 45 degrees and z-18, when ever I do this in the Game camera script like so:

 cameraTarget = new Vector3(target.position.x,transform.position.y,target.position.z-18);

and adjust the camera's angle in the Unity inspector to 45 degrees on the X axis - it all looks great until I move the mouse and the player moves in a completely unexpected and unfavorable way.

Any help? Thanks in advance.