Custom signup page for WP multisite isn't displaying error or success messages

I followed the two links:

1. https://www.chriswiegman.com/2010/02/using-a-custom-wp-signup-page-with-wordpress-mu/
2. https://www.ads-software.com/support/topic/individual-mu-registration-page-possible

And made a custom page with a page template using the same code as wp-signup.php. As there are some files already loaded, I commented out the lines:

//require( dirname(__FILE__) . '/wp-load.php' );

and

//require( dirname( __FILE__ ) . '/wp-blog-header.php' );

And using a mu-plugins\ I used the following code as per @toscho’s suggestion to use the plugins_loaded hook:

function project_redirect_from_default_signup() {
	if( strpos($_SERVER['REQUEST_URI'], 'wp-signup.php')) {
		wp_redirect(network_site_url() .'signup/');
		exit();
	}
}
add_action( 'plugins_loaded', 'project_redirect_from_default_signup' );

But the problem is, with the redirection, the error and success messages are gone away.
How can I display them in my custom page template?