Conditional custom redirect

It should not be tough. As you suggested you can use ACF to add a custom field in post/page and then use the template_redirect action hook to check if the current page’s custom field is a valid URL as well as $_GET[‘redirect’] is true, wp_redirect to that URL.

function my_page_template_redirect()
{
    if( is_url(GET_ACF_FIELD_VALUE( 'CUSTOM_FIELD' )) && !empty($_GET['redirect']) && $_GET['redirect'] == 'true)
    {
        wp_redirect( GET_ACF_FIELD_VALUE( 'CUSTOM_FIELD' ) );
        exit();
    }
}
add_action( 'template_redirect', 'my_page_template_redirect' );