Self rotate issue C#

My code to rotate a ship 180 degrees when it goes out of player range, the code is working in the bottom end but in the high end it's just tuning 5 degrees. Here is the code i'm using:

     private float smooth = 5f;
     public Transform player;
     private float moveSpeed = -5f;
     private Rigidbody2D rb2D;
 
     void Start()
     {
         player = GameObject.FindWithTag("Player").transform;
         if (player == null)
         {
             Debug.Log("Cannot find 'player' object");
         }
         rb2D = GetComponent<Rigidbody2D>();
         rb2D.velocity = transform.up * moveSpeed;
 
     }
 
     void FixedUpdate()
     {
         Vector3 direction = player.position - transform.position;
 
         if (direction.y < -20)
          {
             if (transform.rotation.z == 0f)
             {
                 transform.Rotate(Vector3.back * 0);
             }
             else
             {
                 transform.Rotate(Vector3.back * smooth);
 
             }
             rb2D.velocity = transform.up * (-1 * moveSpeed);
         }
         if (direction.y > 20f)
             {
             if (transform.rotation.z == -1f || transform.rotation.z == 1f)
             {
                 transform.Rotate(Vector3.back * 0);
             }
             else
             {
                 transform.Rotate(Vector3.back * smooth);
             }
 
             rb2D.velocity = transform.up * moveSpeed;
 
         }
 
     }