Enemy kill counter and time limit.

I'm making a game where you kill waves of enemies and depending on which you kill the most harder ones spawn in their place, however, there are a few things I'm not sure how to do...

So there are 3 enemy types by default (TypeA, TypeB, TypeC) -If you kill 3 TypeA, 2 TypeB and 1 TypeC, TypeA is replaced by Type D (A splice of Type 1 and 3)

After each wave, the calculation is done to see which is killed most and least and changes the spawn entities/rates

This is what I have so far:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class WaveSpawner : MonoBehaviour
 {
     public enum SpawnState { SPAWNING, WAITING, COUNTING };
 
     [System.Serializable]
     public class Wave
     {
         public string name;
         public Transform enemy;
         public Transform fireEnemy;
 
         public int enemyCount;
         public float spawnRate;
     }
 
     public int whichEnemy = 0;
     public Wave[] waves;
     private int nextWave = 0;
 
     public Transform[] spawnPoints;
 
     public float timeBetweenWaves = 5f;
     private float waveCountdown;
 
     private float searchCountdown = 1f;
 
     private SpawnState state = SpawnState.COUNTING;
 
     void Start()
     {
         if(spawnPoints.Length == 0)
         {
             Debug.LogError("No Spawn Points Referenced");
         }
         
         waveCountdown = timeBetweenWaves;
 
     }
 
     void Update()
     {
 
         whichEnemy = Random.Range(1,3);
 
         if(state == SpawnState.WAITING)
         {
             if(!EnemyIsAlive())
             {
                 WaveCompleted();
                 
             }
             else
             {
                 return;
             }
         }
 
         if (waveCountdown <= 0)
         {
             if (state != SpawnState.SPAWNING)
             {
                 StartCoroutine(SpawnWave ( waves[nextWave] ) );
             }
         }
         else
         {
             waveCountdown -= Time.deltaTime;
         }
 
     }
 
     void WaveCompleted()  //Add Difficulty Multiplier For More Waves Here
     {
 
         Debug.Log("Wave Completed!");
         
         state = SpawnState.COUNTING;
         waveCountdown = timeBetweenWaves;
 
         if(nextWave + 1 > waves.Length - 1)
         {
             nextWave = 0;
             Debug.Log("ALL WAVES COMPLETE! Looping...");
         }
         else
         {
             nextWave++;
         }
     }
 
 
     bool EnemyIsAlive()
     {
     searchCountdown -= Time.deltaTime;
     if(searchCountdown <= 0f)
     {
         searchCountdown = 1f;
         if(GameObject.FindGameObjectWithTag ("Enemy") == null)
         {
             return false;
         }
     }
         return true;
     }
 
 
     IEnumerator SpawnWave(Wave _wave)
     {
         Debug.Log("Spawning Wave: " + _wave.name);
         state = SpawnState.SPAWNING;
 
         for (int i = 0; i < _wave.enemyCount; i++)
         {
             SpawnEnemy(_wave.enemy);
             yield return new WaitForSeconds(1f / _wave.spawnRate);
         }
 
         state = SpawnState.WAITING;
         yield break;
     }
 
 
 
     void SpawnEnemy (Transform _enemy)
     {
         Debug.Log("Spawning Enemy" + _enemy.name);
 
         if (whichEnemy==1)
         {
             Transform _sp = spawnPoints[ Random.Range(0, spawnPoints.Length)];
             Instantiate(_enemy, _sp.position, _sp.rotation);
         }
         else
         {
             Transform _sp = spawnPoints[ Random.Range(0, spawnPoints.Length)];
             Instantiate(_enemy, _sp.position, _sp.rotation);
         }
 
 
         
         
     }
 
 //Anything beyond here was a WIP 
 
         void EnemiesKilled()
         {
             if(enemyType1.iskilled)
             {
                 enemyType1Counter++;
             }
 
             if(enemyType2.iskilled)
             {
                 enemyType2Counter++;
             }
 
             if(enemyType3.iskilled)
             {
                 enemyType3Counter++;
             }
             
         }
 
 
 
         void SpliceEnemies()
         {
             int enemyType1Counter = 0;
             int enemyType2Counter = 0;
             int enemyType3Counter = 0;
             
             if (enemyType1Counter > enemyType2Counter)
             {
                 Destroy gameobject.enemyType1;
             }
 
         }
 
 
 }

Any help or information would be much appreciated