Update post_meta post_name with the post ID from wp_insert_post after user register

No need for the two functions/hooks – just use the first, with the addition of:

if ( $post_id = wp_insert_post( $user_page ) ) {
    wp_update_post(
        array(
            'ID'        => $post_id,
            'post_name' => $post_id,
        )
    );
}