Trigger not deleting older Event Relations IDs when bulk updating

Question

This is a follow-up question to my previous thread AfterUpdate caused by: System.SObjectException: Field is not writeable EventRelation.RelationId .

Code updates fine for 5 records but when I tried to update 25 records, it keeps older event attendees along with new attendees. It is not properly deleting older event attendees after the update. I know that code is not properly handling bulk records. Don't know how to tweak this so it can handle more records. Thanks.

For example:

Current Attendees: CK, AM, CP

NEW Attendees to update: AA, BB, CC

After Update: CK,AM,CP,AA,BB,CC

But it should be: AA, BB, CC

trigger cloneEventTrigger on Event (after insert, after update) {

if(trigger.isUpdate && cloneEventHelperClass.runOnce) {     
    cloneEventHelperClass.runOnce = false;
   cloneEventHelperClass.onAfterUpdate(trigger.new, trigger.oldmap);
    }     
 }      
}

Helper Class

public class cloneEventHelperClass {
public static boolean runOnce = true;
public static void onAfterUpdate(List<Event> newEvnt, Map<Id, Event>oldMap){  
    // get all ids
    system.debug('After Update cloneEventHelperClass Trigger'); 
    List<Id>eventIds = new List<Id>();
    for(Event evnt : [SELECT Id From Event WHERE id=:newEvnt AND Attendees__c != NULL]){ 
            if (evnt.Id != null){ 
                eventIds.add(evnt.Id);
            }          
        }

    Event[] queryEvent = [
       SELECT Id, Subject, StartDateTime, EndDateTime, OwnerId, Whoid, Whatid,Description,Location, Reason_for_Call_Meeting__c,
        Attendees__c  From Event WHERE id=:eventIds];
        Map<String, Id> initialsToUserId = new Map<String, Id>();
        Set<String> initials = new Set<String>();
        Map<id,User> updateAll= new Map<id, User>();

 // loop through query and add all initials to set 
   for(Event record: queryEvent) {
        initials.addAll(record.Attendees__c.split(';'));
    }

// Pull old initials
    List<String>splitList;
    for(Event evnt : [SELECT Id,Attendees__c From Event WHERE id=:eventIds]){
         if (evnt.Attendees__c != null)
        {

            system.debug('oldEven: ' + oldMap.get(evnt.Id).Attendees__c);
            String multiValues = oldMap.get(evnt.Id).Attendees__c;
            splitList= multiValues.split(';');

        }
    }  

//Remove all existing EventRelation ID if new and old attendees are not same
system.debug('initials:' + initials+'initials size:'+initials.size());
system.debug('splitlist'+ splitList.size());
    If(splitList.size() != initials.size()) {

    System.debug('they are not the same attendees'+ splitList + 'AND '+ initials);
    If(!eventIds.isempty()){
               Map<Id, EventRelation> relations = new Map<Id, EventRelation>([select id from EventRelation where EventId IN: eventIds]);
               System.debug('Size of relation before deletion' + relations.size());
                 If(!relations.isempty()){  
                      delete relations.values();
                 }

               } 


// query for user and their initials and add them to Map. 
    for(User record: [SELECT Id, Initials__c FROM User WHERE Initials__c = :initials AND IsActive = TRUE]) {
      initialsToUserId.put(record.Initials__c, record.Id);
    }
        //Added Debug Statement
        system.debug('@@@initialsToUserId'+initialsToUserId);

// loop through same Event Query to fetch all fields and check all the initial with Id against set. T 

    list <EventRelation> relationsIns = new list <EventRelation>();
    list < Event> eventlist = new list<Event>();

    for(Event record: queryEvent) {
        system.debug('@@@CameIN@@@1');
        record.WhatId = oldMap.get(record.Id).whatId;
        system.debug('@@@record.WhatId '+ record.WhatId);
        eventlist.add(record);
        EventRelation parentRelation = new EventRelation();
             parentRelation.EventId= record.Id;
             parentRelation.RelationId = record.WhatId;
             parentRelation.isWhat = true;
             parentRelation.isParent = true;
             parentRelation.isInvitee = false;
        relationsINS.add(parentRelation);  
        for(String initial: record.Attendees__c.split(';')) {
            system.debug('@@@CameIN@@@2');
            Id attendeeId = initialsToUserId.get(initial);
            system.debug('@@@'+attendeeId +'@@@initials'+initial);
            if(attendeeId != null && record.OwnerId != attendeeId) {
             system.debug('@@@CameIN@@@3');

             EventRelation e = new EventRelation();
             e.EventId= record.Id;
             e.RelationId = attendeeId;
             relationsINS.add(e);  

          }    
        }
      }// end of query
        system.debug('Trigger Ran FINE!!!!!'); 
        system.debug('relations To Insert: '+relationsINS); 
        list <EventRelation> relations1 =[select id from EventRelation where EventId IN: eventIds];
        System.debug('Size of relation after deletion' + relations1.size());
        //Database.insert(relationsINS,false);
        //update eventlist;

        insert relationsINS; 

      } // End of IF Statement
    } //End of onAfterInsert Trigger
}