Extend the Database Driver Class (in DB_driver)

I need replication to be automatic in the framework (someone does a write function, we need it to automatically select the master table and perform the query with a set comment in union with the PHP plug in we're using to accomplish the correct database to query from)

I've modified the DB_driver.php file in CI 3.1.5, however, I'd like to really do this outside of the core file.

Quick notes on what we did:

The config database.php file has two lines in it in the $db['default'] array

PHP Code:

'replication' => TRUE,
 'replication_text' => '/*ms=master*/' 


The DB_driver.php file has an added function (swipped from CI4's DB files)

PHP Code:

//--------------------------------------------------------------------

 /**
 * Determines if the statement is a write-type query or not.
 *
 * @return bool
 */
 public function isWriteType($sql): bool
 {
     return (bool)preg_match(
     '/^\s*"?(SET|INSERT|UPDATE|DELETE|REPLACE|CREATE|DROP|TRUNCATE|LOAD|COPY|ALTER|RENAME|GRANT|REVOKE|LOCK|UNLOCK|REINDEX)\s/i',
 $sql);
 } 


And then I modified the simple_query function like so:

PHP Code:

public function simple_query($sql)
    {
        if ( ! $this->conn_id)
        {
            if ( ! $this->initialize())
            {
                return FALSE;
            }
        }
        if ($this->replication) {
            if ($this->isWriteType($sql)) {
                $sql = $this->replication_text."
                ".$sql;                
            }
        }
        return $this->_execute($sql);
    } 


I added a MY_DB_drive.php file in the core folder hoping it would work (pulling out the above changes and making it in to an extended class) like so:

PHP Code:

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class MY_DB_drive extends DB_driver {
    /**
     * Constructor
     */
    public function __construct()
    {
        parent::__construct();
    }
    
    public function simple_query($sql)
    {
        if ( ! $this->conn_id)
        {
            if ( ! $this->initialize())
            {
                return FALSE;
            }
        }
        if ($this->replication) {
            if ($this->isWriteType($sql)) {
                $sql = $this->replication_text."
                ".$sql;                
            }
        }
        return $this->_execute($sql);
    }
    
    //--------------------------------------------------------------------

    /**
     * Determines if the statement is a write-type query or not.
     *
     * @return bool
     */
    public function isWriteType($sql): bool
    {
        return (bool)preg_match(
            '/^\s*"?(SET|INSERT|UPDATE|DELETE|REPLACE|CREATE|DROP|TRUNCATE|LOAD|COPY|ALTER|RENAME|GRANT|REVOKE|LOCK|UNLOCK|REINDEX)\s/i',
            $sql);
    }
} 


But that didn't work.  Any pointers on getting what I'd like done without editing the core files?