Add slug to default post permalinks ONLY

Literally found the answer 10 minutes after posting this question and spending a couple of hours on it before asking. Simply hook your function to post_link! 🤦

public function __construct() {
  add_action('post_link', [$this, 'post_links'], 10, 2);
}

public function post_links($link, $post) {
  if ($post->post_type === 'post') {
    return site_url('slug/'. $post->post_name .'-'. $post->ID);
  } else {
    return $link;
  }
}