Using two different DB users on one WP install

I’m curious as to whether the following is possible as there are concerns that the WP site in question may be hacked via the front end & through a plugin..

1. We want to have the front-end connect via a Read-Only DB user
2. We want the admin area to have Read/Write access but have this URL protected by the server firewall rule that only allows internal traffic

I’m open to other suggestions

I have setup WP instance with 2 different database, one for read only and other for admin purpose.

But once the admin make the changes, then the ADMIN-DB should be copied and added to SLAVE-DB server.

// check if url contains wp-login or wp-admin and then create DB configuration for Master DB else Slave DB.
$url = "http://".$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI'];
if ( false !== strpos( $url, 'wp-admin' ) || false !== strpos( $url, 'wp-login' ) ) {
    define( 'DB_NAME', 'SLAVE_DB_NAME' );
    define( 'DB_USER', 'SLAVE_DB_USER' );
    define( 'DB_PASSWORD', 'SLAVE_DB_PASSWORD' );
    define( 'DB_HOST', 'SLAVE_DB_HOST' );
} else {
    define( 'DB_NAME', 'MASTER_DB_NAME' );
    define( 'DB_USER', 'MASTER_DB_USER' );
    define( 'DB_PASSWORD', 'MASTER_DB_PASSWORD' );
    define( 'DB_HOST', 'MASTER_DB_HOST' );

}