Help with database load design

I've been thru the same case just like you, years ago.
here's what i did


create a "core" model, let's just call it "Coremodel" 

PHP Code:

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

class Coremodel extends CI_Model {
    
    public static $USR_DB;
    
    public function __construct()
    {
        parent::__construct();
        
        self::$USR_DB['user_1']     = $this->load->database('user_1',true);
        self::$USR_DB['user_2']     = $this->load->database('user_2',true);
        self::$USR_DB['user_3']     = $this->load->database('user_3',true);
        // and so'on
    }

} 


this model will act as a core parent for all models that you'll create later on.notice that i put a public-static variable inside this 'core' model.


later on, when you try creating models for each of your user's custom db connection,
just extend from our "Coremodel" 

PHP Code:

class Model_user1 extends Coremodel {
    
    private $_DB;
    
    function __construct() { 
        parent::__construct();
        $this->_DB     = parent::$USR_DB['user_1'];
    }
    
    public function get($id)
    {
        return $this->_DB->get_where("table_name", ['id'=>$id]);
    } 


notice that i put a private var "_DB" and then retrieve our db connection instance from "Coremodel",then put it in private var "_DB". 


lastly, make sure that our "Coremodel" is autoloaded from autoload config.

PHP Code:

$autoload['model'] = array('coremodel'); 


now you have all of your model's connected accordingly,
without haveing to create a new connection for each models.

hope this make sense.