composer setup with CMB2 in project file

composer.json

{
    "name": "client/project",
    "description": "Client Name",
    "authors": [],
    "type": "project",
    "repositories": [
        {
            "type": "composer",
            "url": "https://wpackagist.org"
        },
        {
            "type": "package",
            "package": {
                "name": "johnbillion/extended-cpts",
                "version": "4.2.0",
                "type": "wordpress-muplugin",
                "source": {
                    "type": "git",
                    "url": "https://github.com/johnbillion/extended-cpts",
                    "reference": "tags/4.2.0"
                },
                "require": {
                    "composer/installers": "~1.0"
                },
                "autoload": {
                    "files": [
                        "extended-cpts.php"
                    ]
                }
            }
        },
        {
            "type": "package",
            "package": {
                "name": "cmb2/cmb2",
                "version": "2.4.2",
                "type": "wordpress-muplugin",
                "source": {
                    "type": "git",
                    "url": "https://github.com/cmb2/cmb2",
                    "reference": "tags/v2.4.2"
                }
            }
        }
    ],
    "prefer-stable": true,
    "minimum-stability": "dev",
    "require": {
        "wpackagist-plugin/gutenberg": "~4.1",
        "johnbillion/extended-cpts": "^4.2",
        "wpackagist-plugin/wp-redis": "^0.7",
        "cmb2/cmb2": "^2.4.2"
    },
    "require-dev": {
        "wpackagist-plugin/debug-bar": "9999999-dev",
        "wpackagist-plugin/debug-bar-extender": "9999999-dev",
        "wpackagist-plugin/debug-bar-cron": "9999999-dev",
        "wpackagist-plugin/developer": "9999999-dev",
        "wpackagist-plugin/rewrite-rules-inspector": "9999999-dev",
        "wpackagist-plugin/log-deprecated-notices": "9999999-dev",
        "wpackagist-plugin/query-monitor": "9999999-dev",
        "phpunit/phpunit": "~6.5",
        "10up/wp_mock": "~0.3",
        "squizlabs/php_codesniffer": "~3.3",
        "humanmade/coding-standards": "~0.5",
        "dealerdirect/phpcodesniffer-composer-installer": "^0.4.4"
    },
    "extra": {
        "installer-paths": {
            "plugins/{$name}/": ["type:wordpress-plugin"],
            "mu-plugins/{$name}/": ["type:wordpress-muplugin"]
        }
    }
}

loader.php

<?php
/*
Plugin Name: Plugin Loader
Description: Loads the MU plugins required to run the site. This file exists in wp-content/mu-plugins and it is assumed 
             that the composer packages we're including are also stored here.
Version: 1.0
*/

if ( defined( 'WP_INSTALLING' ) && WP_INSTALLING ) {
	return;
}

// Plugins to be loaded for any site
$global_mu_plugins = [
	'extended-cpts/extended-cpts.php',
	'cmb2/init.php',
];

// Load the plugin files
foreach ( $global_mu_plugins as $file ) {
	if ( file_exists( WPMU_PLUGIN_DIR . '/' . $file ) ) {
		require_once WPMU_PLUGIN_DIR . '/' . $file;
	}
}