How to set Game Object to spawn after a certain timing After Level Loads?

Hello guys! I have a script that spawns my kill objects. I want my kill objects to spawn after 5 secs, 20secs, 60secs, 90secs so that I will have a total of 5 kill objects in the game after 90 secs. Any idea how to do that?

The following is my kill object spawning script :

 using UnityEngine;
 using System.Collections;
 
 public class KillObjectGenerator : MonoBehaviour
 {
 
     public float randomTimeDelay;
     private float randomTimeDelay2;
     private float randomTimeDelay3;
     private float randomTimeDelay4;
     private float randomTimeDelay5;
     public GameObject cloud;
     public GameObject lightning;
     private float randomX;
     private Vector2 newPositionL;
     private Vector2 newPositionC;
     private Vector2 newPositionL1;
     private Vector2 newPositionC1;
     private Vector2 newPositionL2;
     private Vector2 newPositionC2;
     private Vector2 newPositionL3;
     private Vector2 newPositionC3;
     private Vector2 newPositionL4;
     private Vector2 newPositionC4;
 
     void Start()
     {
         randomTimeDelay = Random.Range(3f, 6f);
         randomTimeDelay2 = Random.Range(3f, 6f);
         randomTimeDelay3 = Random.Range(3f, 6f);
         randomTimeDelay4 = Random.Range(3f, 6f);
         randomTimeDelay5 = Random.Range(3f, 6f);
         // Start calling the Spawn function repeatedly after a delay.
         InvokeRepeating("GenerateFirst", randomTimeDelay, 2.0f);
 
         if (Time.frameCount > (5 * 30))
         {
             InvokeRepeating("GenerateSecond", randomTimeDelay2, 2.0f);
             Debug.Log("2nd one coming to get you!");
 
             if (Time.frameCount > (20 * 30))
             {
                 InvokeRepeating("GenerateThird", randomTimeDelay3, 2.0f);
                 Debug.Log("3rd one coming to get you!");
 
                 if (Time.frameCount > (60 * 30))
                 {
                     InvokeRepeating("GenerateForth", randomTimeDelay4, 2.0f);
                     Debug.Log("4th one coming to get you!");
 
                     if (Time.frameCount > (90 * 30))
                     {
                         InvokeRepeating("GenerateFifth", randomTimeDelay5, 2.0f);
                         Debug.Log("5th one coming to get you!");
                     }
                 }
             } 
         }    
     }
 
     void GenerateFirst()
     {
         float randomX = Random.Range(-5.25f, 6.0f);
         newPositionL = new Vector2(randomX, 1.5f);
         newPositionC = new Vector2(randomX, 0.23f);
         Instantiate(cloud, newPositionC, Quaternion.identity);
         Instantiate(lightning, newPositionL, Quaternion.identity);
         Debug.Log("Spawned a kill object!");
     }
 
     void GenerateSecond()
     {
         float randomX1 = Random.Range(-5.25f, 6.0f);
         newPositionL1 = new Vector2(randomX1, 1.5f);
         newPositionC1 = new Vector2(randomX1, 0.23f);
         Instantiate(cloud, newPositionC1, Quaternion.identity);
         Instantiate(lightning, newPositionL1, Quaternion.identity);
     }
 
     void GenerateThird()
     {
         float randomX2 = Random.Range(-5.25f, 6.0f);
         newPositionL2 = new Vector2(randomX2, 1.5f);
         newPositionC2 = new Vector2(randomX2, 0.23f);
         Instantiate(cloud, newPositionC2, Quaternion.identity);
         Instantiate(lightning, newPositionL2, Quaternion.identity);
     }
 
     void GenerateForth()
     {
         float randomX3 = Random.Range(-5.25f, 6.0f);
         newPositionL3 = new Vector2(randomX3, 1.5f);
         newPositionC3 = new Vector2(randomX3, 0.23f);
         Instantiate(cloud, newPositionC3, Quaternion.identity);
         Instantiate(lightning, newPositionL3, Quaternion.identity);
     }
 
     void GenerateFifth()
     {
         float randomX4 = Random.Range(-5.25f, 6.0f);
         newPositionL4 = new Vector2(randomX4, 1.5f);
         newPositionC4 = new Vector2(randomX4, 0.23f);
         Instantiate(cloud, newPositionC4, Quaternion.identity);
         Instantiate(lightning, newPositionL4, Quaternion.identity);
     }
 
 }