I am a newbie when it comes to Apex development, but I am trying to deploy a trigger that sends a task and an email when an opportunity is marked as won. Half of the trigger is working properly, but the most important half is not. Any help is greatly appreciated!

I have a custom field on the Account level named "Customer_Success_Owner_Name__c". The field is a formula that takes another field and returns the name "Brandon", "Kyle", or "Oz". Based on the name in that field, the trigger should create a task and send an email to the correct user, as well as do the same for our finance officer.

The "Open a Payment Record" task is sending correctly. The "Open a Subscription" is acting sporadic and not sending to the correct user when it does actually send.
 

/*Open a Task on Account on a closed won Opportunity */

trigger OpenAccountTask on Opportunity (after update){
    
    List<Id> AccountId = new List<Id>();
    
    for(Integer i = 0; i < trigger.size; i++){
        if(trigger.new[i].StageName == 'Closed Won' && trigger.new[i].StageName != trigger.old[i].StageName)
            AccountId.add(trigger.new[i].AccountId);
    }
    
    if(!AccountId.isEmpty()){
        Task[] tasksToInsert = new Task[]{};
        Map<Id,Account> GetAccountById = new Map<Id,Account>();
        for(Account a : [SELECT Id, Name, Customer_Success_Owner_Name__c, OwnerId FROM Account WHERE Id IN :AccountId]){
            GetAccountById.put(a.Id,a);
        }
        for(Integer i = 0; i < trigger.size; i++){

            /*US Task for Brandon */
            if(trigger.new[i].StageName == 'Closed Won' && trigger.new[i].StageName != trigger.old[i].StageName && (GetAccountById.get(trigger.new[i].AccountId).Customer_Success_Owner_Name__c == 'Brandon')){
                tasksToInsert.add(new Task(WhatId=trigger.new[i].AccountId, OwnerID='005700000023zFI', Subject='Open a Subscription',ActivityDate=Date.Today()));
                tasksToInsert.add(new Task(WhatId=trigger.new[i].AccountId, OwnerID='005700000026xbg', Subject='Open a Payment record',ActivityDate=Date.Today()));
                Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();              
                mail.setTargetObjectId('005700000023zFI');
                mail.setSubject('Open a Subscription for ' + GetAccountById.get(trigger.new[i].AccountId).Name);
                mail.setPlainTextBody('You can view the details here:\n' + URL.getSalesforceBaseUrl().toExternalForm() + '/' + trigger.new[i].AccountId);
                mail.setSaveAsActivity(false);
                Messaging.SingleEmailMessage mail2 = new Messaging.SingleEmailMessage();
                mail2.setTargetObjectId('005700000026xbg');
                mail2.setSaveAsActivity(false);
                mail2.setSubject('Open a Payment record for ' + GetAccountById.get(trigger.new[i].AccountId).Name);
                mail2.setPlainTextBody('You can view the details here:\n' + URL.getSalesforceBaseUrl().toExternalForm() + '/' + trigger.new[i].AccountId);
                Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail,mail2 });
            }

            /*US & EMEA Task for Kyle*/
            else if(trigger.new[i].StageName == 'Closed Won' && trigger.new[i].StageName != trigger.old[i].StageName && (GetAccountById.get(trigger.new[i].AccountId).Customer_Success_Owner_Name__c == 'Kyle')){
                tasksToInsert.add(new Task(WhatId=trigger.new[i].AccountId, OwnerID='00570000003DXJb', Subject='Open a Subscription',ActivityDate=Date.Today()));
                tasksToInsert.add(new Task(WhatId=trigger.new[i].AccountId, OwnerID='005700000026xbg', Subject='Open a Payment record',ActivityDate=Date.Today()));
                Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
                mail.setTargetObjectId('00570000003DXJb');
                mail.setSaveAsActivity(false);
                mail.setSubject('Open a Subscription for ' + GetAccountById.get(trigger.new[i].AccountId).Name);
                mail.setPlainTextBody('You can view the details here:\n' + URL.getSalesforceBaseUrl().toExternalForm() + '/' + trigger.new[i].AccountId);
                Messaging.SingleEmailMessage mail2 = new Messaging.SingleEmailMessage();
                mail2.setTargetObjectId('005700000026xbg');
                mail2.setSaveAsActivity(false);
                mail2.setSubject('Open a Payment record for ' + GetAccountById.get(trigger.new[i].AccountId).Name);
                mail2.setPlainTextBody('You can view the details here:\n' + URL.getSalesforceBaseUrl().toExternalForm() + '/' + trigger.new[i].AccountId);
                Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail,mail2 });
            }       

            /*AP & Enterprise Task for Oz*/
            else if(trigger.new[i].StageName == 'Closed Won' && trigger.new[i].StageName != trigger.old[i].StageName && (GetAccountById.get(trigger.new[i].AccountId).Customer_Success_Owner_Name__c == 'Oz')){
                tasksToInsert.add(new Task(WhatId=trigger.new[i].AccountId, OwnerID='00570000001XWfY', Subject='Open a Subscription',ActivityDate=Date.Today()));
                tasksToInsert.add(new Task(WhatId=trigger.new[i].AccountId, OwnerID='005700000026xbg', Subject='Open a Payment record',ActivityDate=Date.Today()));
                Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
                mail.setTargetObjectId('00570000001XWfY');
                mail.setSaveAsActivity(false);
                mail.setSubject('Open a Subscription for ' + GetAccountById.get(trigger.new[i].AccountId).Name);
                mail.setPlainTextBody('You can view the details here:\n' + URL.getSalesforceBaseUrl().toExternalForm() + '/' + trigger.new[i].AccountId);
                Messaging.SingleEmailMessage mail2 = new Messaging.SingleEmailMessage();
                mail2.setTargetObjectId('005700000026xbg');
                mail2.setSaveAsActivity(false);
                mail2.setSubject('Open a Payment record for ' + GetAccountById.get(trigger.new[i].AccountId).Name);
                mail2.setPlainTextBody('You can view the details here:\n' + URL.getSalesforceBaseUrl().toExternalForm() + '/' + trigger.new[i].AccountId);
                Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail,mail2 });
            }
        }
        if(!tasksToInsert.isEmpty())
            Database.insert(tasksToInsert);     
    }
}