IMO、プラグインは必要な作業が少なく、特定のケースにより適しています。
<?php
/*
Plugin Name: Disalbe Frontend
Description:  Disable the frontend interface of the website, leave only CMS and REST API
Author: Nikola Mihyalov
Version: 1.0
*/
add_action('init', 'redirect_to_backend');
function redirect_to_backend() {
    if(
        !is_admin() &&
        !is_wplogin() &&
        !is_rest()
    ) {
    wp_redirect(site_url('wp-admin'));
    exit();
  }
}
if (!function_exists('is_rest')) {
    /**
     * Checks if the current request is a WP REST API request.
     * 
     * Case #1: After WP_REST_Request initialisation
     * Case #2: Support "plain" permalink settings
     * Case #3: URL Path begins with wp-json/ (your REST prefix)
     *          Also supports WP installations in subfolders
     * 
     * @returns boolean
     * @author matzeeable
     */
    function is_rest() {
        $prefix = rest_get_url_prefix( );
        if (defined('REST_REQUEST') && REST_REQUEST // (#1)
            || isset($_GET['rest_route']) // (#2)
                && strpos( trim( $_GET['rest_route'], '\\/' ), $prefix , 0 ) === 0)
            return true;
        // (#3)
        $rest_url = wp_parse_url( site_url( $prefix ) );
        $current_url = wp_parse_url( add_query_arg( array( ) ) );
        return strpos( $current_url['path'], $rest_url['path'], 0 ) === 0;
    }
}
function is_wplogin(){
    $ABSPATH_MY = str_replace(array('\\','/'), DIRECTORY_SEPARATOR, ABSPATH);
    return ((in_array($ABSPATH_MY.'wp-login.php', get_included_files()) || in_array($ABSPATH_MY.'wp-register.php', get_included_files()) ) || (isset($_GLOBALS['pagenow']) && $GLOBALS['pagenow'] === 'wp-login.php') || $_SERVER['PHP_SELF']== '/wp-login.php');
}