How do I hide elements from a child class embedded in a List on the Inspector in the CustomEditor?

There is a boolean for each 'Quest' class made which, when toggled, is supposed to hide/reveal yet another List of class 'Objective_Killing' shown here https://imgur.com/a/0ew8tQO

But I have no luck in trying to make the functionality work because I can't find a way to affect elements of a class property within another class property. Someone please help me with this.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;

 public class NPCQuestList : MonoBehaviour
 {
     public List<GameObject> questObjList = new List<GameObject>();

     public Quest[] questsAvailable;
     }
 }

 [System.Serializable]
 public class Quest
 {
     public bool inProgress;
     public string questName;
     public string description;

     public bool has_killing_objective;
     public Objective_Killing[] killing_Objective;
 }

 [System.Serializable]
 public class Objective_Killing
 {
     public string target;
     public int currentkillcount;
     public int maxkillcount;
     public bool objectiveComplete;
 }

CustomEditor script

 [CustomEditor(typeof(NPCQuestList))]
 public class QuestEditor : Editor
 {
     bool questObjListToggle;
     bool questListToggle;

     public override void OnInspectorGUI()
     {
     
         SerializedProperty sp = serializedObject.FindProperty("questsAvailable");
         SerializedProperty killingObjBoolSP = sp.GetArrayElementAtIndex(0).FindPropertyRelative("has_killing_objective");
         NPCQuestList questScript = (NPCQuestList) target;

         SerializedObject so = sp.GetArrayElementAtIndex(0).serializedObject;

         questObjListToggle = GUILayout.Toggle(questObjListToggle, "Enable Quest Objective List");
         questListToggle = GUILayout.Toggle(questListToggle, "Enable Quest List");
         List<string> excludedElements = new List<string>();

         if (!questObjListToggle) { excludedElements.Add("questObjList"); }
         if (!questListToggle) { excludedElements.Add("questsAvailable"); }
         if (!killingObjBoolSP.boolValue) { excludedElements.Add("killing_Objective"); }

         DrawPropertiesExcluding(serializedObject, excludedElements.ToArray());
         serializedObject.ApplyModifiedProperties();
     }
 }