How do I destroy past obstacles in an endless runner?

I have obstacles spawning ahead of the runner and want them to be destroyed after the runner passes them. Here is the instantiation code attached to an empty game object:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class Spawner : MonoBehaviour
 {
 
     public GameObject[] obstaclePatterns;
     public Transform player;
     private float timeBtwSpawn;
     public float startTimeBtwSpawn;
     public float obstacleSpawnDistance;
     public float obstacleSpDMin;
     public float obstacleSpDMax;
     public float decreaseTime;
     public float minTime = 0.65f;
     public float obstacleHeightMin = 10f;
     public float obstacleHeightMax = 15f;
 
 // makes obstacles randomly ahead of player
 
     private void FixedUpdate() {
         if (timeBtwSpawn <= 0) {
             obstacleSpawnDistance = Random.Range(obstacleSpDMin, obstacleSpDMax);
             int rand = Random.Range(0, obstaclePatterns.Length);
             Instantiate(obstaclePatterns[rand], new Vector3(obstaclePatterns[rand].transform.position.x, Random.Range(obstacleHeightMin, obstacleHeightMax), player.position.z + obstacleSpawnDistance), Quaternion.identity);
             rand = Random.Range(0, obstaclePatterns.Length);
             Instantiate(obstaclePatterns[rand], new Vector3(obstaclePatterns[rand].transform.position.x, Random.Range(obstacleHeightMin, obstacleHeightMax), player.position.z + obstacleSpawnDistance), Quaternion.identity);
             rand = Random.Range(0, obstaclePatterns.Length);
             Instantiate(obstaclePatterns[rand], new Vector3(obstaclePatterns[rand].transform.position.x, Random.Range(obstacleHeightMin, obstacleHeightMax), player.position.z + obstacleSpawnDistance), Quaternion.identity);
             timeBtwSpawn = startTimeBtwSpawn;
             if (startTimeBtwSpawn > minTime) {
                 startTimeBtwSpawn -= decreaseTime;
             }
         }
         else {
             timeBtwSpawn -= Time.deltaTime; 
         }
     }
 }
 

And here is the destroy code attached to my obstacle prefab:

 using UnityEngine;
 
 public class ObjectDestruction : MonoBehaviour {
 
     public Transform player;
 
     // Update is called once per frame
     void FixedUpdate()
     {
         if (transform.position.z < player.position.z) {
             Destroy(gameObject);
             Debug.Log("test");
         }
     }
 }
 

Thank you so much in advance!!