Problems with basic enemyAI on sphereical world

I have a spherical world using basic faux gravity and the player works perfectely but you can't NavMesh bake a 3d object like that so I made my own very bad script. I left out the y in the vector because otherwise the enmy would try to go into the ground at certain angles. But this causes strange behaviours: the enemy goes to the wrong spot where the z and x coördinates are the same but the y negative and when the enmy does reach destination it circles the player and then shoots of to the other coördinate.

Script:

 public Transform player;
 private Rigidbody rb;
 public Vector3 moveDirection;
 public float speed = 20;

 // Use this for initialization
 void Start ()
 {
     player = GameObject.FindGameObjectWithTag("Player").transform;
     rb = GetComponent<Rigidbody>();
 }
 
 // Update is called once per frame
 void Update ()
 {
     moveDirection = new Vector3(player.transform.position.x - rb.transform.position.x, 0, player.transform.position.z - rb.transform.position.z).normalized;
 }

 private void FixedUpdate()
 {
     rb.MovePosition(rb.position + transform.TransformDirection(moveDirection) * speed * Time.deltaTime);
 }