stop customize.php redirect to login page if admin is not logged in

to redirect all admin pages to home page when the user is not connected, try that

add_filter("login_url", function ($login_url, $redirect, $force_reauth) {

    if (!is_user_logged_in()) {
        $login_url = home_url("https://wordpress.stackexchange.com/");
    }


    return $login_url;

}, 10, 3);

If you want to redirect only on the page “customize.php”, you can add the condition at the “if”.