Statamic v3 navigation structure custom API

Controller.php

<?php
use Statamic\Http\Controllers\Controller as BaseController;
use Statamic\Facades\Structure;
use Statamic\Structures\Page;

class Controller extends BaseController
{
    public function navigations()
    {
        $structure = Structure::findByHandle("nav_main");
        $trees = $structure->trees();
        $defaultSiteTree = $trees->get('default');
        $pageTree["main_navigation"] = $defaultSiteTree->pages()->all()->map(function(Page $page) {
            $subpages = $page->pages()->all()->map(function(Page $sub_page) {
                $title = $sub_page->augmented()->get("navigation_title")->value() === NULL ? $sub_page->title() : $sub_page->augmented()->get("navigation_title");
                return [
                    "title" => $title,
                    "href" => $sub_page->uri(),
                    "showDesktop" => $sub_page->augmented()->get("nav_show_desktop")->value() === NULL ? true : $sub_page->augmented()->get("nav_show_desktop")->value(),
                    "showMobile" => $sub_page->augmented()->get("nav_show_mobile")->value() === NULL ? true : $sub_page->augmented()->get("nav_show_mobile")->value()
                ];
            });
            $navstruct["title"] = $page->augmented()->get("navigation_title")->value() === NULL ? $page->title() : $page->augmented()->get("navigation_title");
            $navstruct["showDesktop"] = $page->augmented()->get("nav_show_desktop")->value() === NULL ? true : $page->augmented()->get("nav_show_desktop")->value();
            $navstruct["showMobile"] = $page->augmented()->get("nav_show_mobile")->value() === NULL ? true : $page->augmented()->get("nav_show_mobile")->value();
            $navstruct["href"] = $page->uri();
            $navstruct["children"] = $subpages;
            return $navstruct;
        });
        return  $pageTree;
    }
}