WordPress - Get child pages and grandchild page list. Get ancestor list when on grandchild page.

gobble-tier.php

<?php

if(is_page())
{
    //Assuming current working page is the parent
    //
    $the_parent_id = $post->ID;

    //Otherwise get the greatest ancestor id
    //
    
    if(! empty($post->ancestors))
    {
        $the_parent_key = max(array_keys($post->ancestors));

        $the_parent_id = $post->ancestors[$the_parent_key];
    }
	
    //First, get all of the pages
    //
    $all_wp_pages = get_pages();

    //Get all of the children relative to the greatest ancestor
    //
    $page_services_children = get_page_children($the_parent_id, $all_wp_pages);
    
    $page_walk_defaults = array();
    $page_walk_defaults['depth'] = 3;
    $page_walk_defaults['show_date'] = '';
    $page_walk_defaults['date_format'] = get_option('date_format');
    $page_walk_defaults['child_of'] = 0;
    $page_walk_defaults['exclude'] = '';
    $page_walk_defaults['title_li'] = '';
    $page_walk_defaults['echo'] = 0;
    $page_walk_defaults['authors'] = '';
    $page_walk_defaults['sort_column'] = 'post_date';
    $page_walk_defaults['link_before'] = '';
    $page_walk_defaults['link_after'] = '';
    $page_walk_defaults['walker'] = '';

    $output = '';

    $output .= '<ul>';
    
    $output .= '<li>'.get_the_title($the_parent_id).'</li>';
    
    $output .= walk_page_tree($page_services_children, $page_walk_defaults['depth'], $the_parent_id, $page_walk_defaults);

    $output .= '</ul>';

    $output = apply_filters('wp_list_pages', $output, $page_walk_defaults);

    echo $output;
}

?>

I'm also looking to add the current_page_item to this.