Despawn Parts of Level After They Leave a Certain Range

I have experience with unity and c# but needed a little help with this one.

How can I despawn level parts after I have already used them during my game. I'm using a list to keep track of the spawned parts but I am having trouble keeping track deleting them once they have gotten out of a certain range from the player.

This is my level generator script so far.

Any help is appreciated. Thanks in advance.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class LevelGenerator : MonoBehaviour {
 
     // Amount that checks is player is close enough to spawn the next platforms.
     private const float PLAYER_DISTANCE_SPAWN_LEVEL_PART = 200f;
     // Amount that checks is player is close enough to despawn the previous platforms.
     private const float PLAYER_DISTANCE_DELETE_LEVEL_PART = 200f;
 
     [Header("Level Technicalities")]
     [SerializeField] private Transform levelPart_Start;
     [SerializeField] private List<Transform> levelPartList;
     [SerializeField] private MobilePlayerMovement player;
     [SerializeField] private List<GameObject> levelListForDespawning;
 
     [Header("Positional Values / Offsets")]
     public float xPos = 0;
     public float yPos = -1;
     public float zPos = 35;

     private bool isStart;
     private Vector3 lastEndPosition;
     private Vector3 firstPlatformPosition;
     private bool isSpawningSimilarPart;
     int chosenLevelPartIndex;
     int lastLevelIndex;
 
 
     private void Awake() 
     {
         isSpawningSimilarPart = false;
 
         lastEndPosition = levelPart_Start.Find("EndPosition").position;
         firstPlatformPosition = levelPart_Start.Find("StartPlatform").position;
 
         int startingSpawnLevelParts = 5;
         isStart = true;
         for (int i = 0; i < startingSpawnLevelParts; i++) {
             SpawnLevelPart();
         }
     }
    
     private void Update() 
     {
         if (Vector3.Distance(player.transform.position, lastEndPosition) < PLAYER_DISTANCE_SPAWN_LEVEL_PART) 
         {
             SpawnLevelPart();
         }
 
         if (Vector3.Distance(player.transform.position, firstPlatformPosition) > PLAYER_DISTANCE_DELETE_LEVEL_PART) 
         {
            DespawnLevelPart(levelListForDespawning[0]);
         }
     }
 
     private void SpawnLevelPart() 
     {
         if(isStart == true)
         {
             chosenLevelPartIndex = 0;
             isStart = false;
         } else {
             do {
                 chosenLevelPartIndex = Random.Range(0, levelPartList.Count);
             }
             while(chosenLevelPartIndex == lastLevelIndex);
         }
  
         lastLevelIndex = chosenLevelPartIndex;
         Transform chosenLevelPart = levelPartList[chosenLevelPartIndex];
         Vector3 offset = new Vector3(xPos, yPos, zPos);
         /*
         if (chosenLevelPartIndex == 0) // The Tunnel
         {
             isSpawningSimilarPart = true;  
         }
         else if (chosenLevelPartIndex == 1) // Block-Box HARD
         {
             isSpawningSimilarPart = true;  
         }
         else if (chosenLevelPartIndex == 2) // Block-Box Easy
         {
             isSpawningSimilarPart = true;            
         } 
         else if (chosenLevelPartIndex == 3) // MiniMaze Left
         {
             isSpawningSimilarPart = false;
         }  
         else if (chosenLevelPartIndex == 4) // MiniMaze Right
         {
             isSpawningSimilarPart = true;
         } 
         else if (chosenLevelPartIndex == 5) // Slim Pathways
         {
             isSpawningSimilarPart = false;
         }*/
 
         Transform lastLevelPartTransform = SpawnLevelPart(chosenLevelPart, lastEndPosition + offset);
         levelListForDespawning.Add(lastLevelPartTransform.gameObject);
         lastEndPosition = lastLevelPartTransform.Find("EndPosition").position;
         firstPlatformPosition = levelListForDespawning[0].transform.position;
 
         lastLevelIndex = chosenLevelPartIndex;   
     }
 
 
     private Transform SpawnLevelPart(Transform levelPart, Vector3 spawnPosition) 
     {
         Transform levelPartTransform = Instantiate(levelPart, spawnPosition, Quaternion.identity);
         return levelPartTransform;
     }
 
 
     private void DespawnLevelPart(GameObject levelPartToDespawn)
     {
         //Destroy(levelPartToDespawn);
         Debug.Log(levelPartToDespawn);
         levelListForDespawning.Remove(levelPartToDespawn);
     }
 }