How to save a thumbnail ID and make sure the attached Post ID is referenced in Media Library

functions.php

<?php
add_action('save_post', 'relate_post_to_thumb');

function relate_post_to_thumb($post_id) {
        
    //does this post have a URL / thumbnail
    $thumb_id = get_post_thumbnail_id($post_id);
    
    //if thumb found relate to post
    if($thumb_id){
        //does our thumb have a post parent
        if(!has_post_parent($thumb_id)){
            wp_update_post(
                array(
                    'ID' => $thumb_id, 
                    'post_parent' => $post_id
                )
            );
        };
    }
}