opnform/app/Http/Controllers/SitemapController.php

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use Spatie\Sitemap\Sitemap;
use Spatie\Sitemap\Tags\Url;
use App\Models\Template;

class SitemapController extends Controller
{

    public function index(Request $request)
    {
        return [
            ...$this->getTemplatesUrls()
        ];
    }

    private function getTemplatesUrls()
    {
        $urls = [];
        Template::where('publicly_listed', true)->chunk(100, function ($templates) use (&$urls) {
            foreach ($templates as $template) {
                $urls[] = [
                    'loc' => '/templates/' . $template->slug
                ];
            }
        });
        return $urls;
    }
}