How to assign a custom layout to a page according to its URL alias in Drupal 8

For the past couple of weeks I've been working as a contractor for an NYC-based NGO that's using Drupal 8 to power their site. My first task was to move their styleguide from a design done in Figma to actual components built with HTML and CSS inside Drupal, but there was a trick: the styleguide had to be set up as some sort of general type content and all I was able to customize was the URL alias.

I went on and duplicated the page.html.twig in our subtheme in order to customize that and work there, later renamed the file to page--styleguide.html.twig. After that I created all related content and used a URL alias that was styleguide happily knowing that (as stated by Drupal's documentation) it would pick up the custom layout automatically without the need of indicating anything.

Narrator: the custom layout wasn't picked up by Drupal

I wasn't able to find why the custom layout was not being used, so I decided to do some googling and found a useful function to suggest Drupal which layout to use. After some tweaking I ended up with the following snippet (placed inside my .theme file)

// Set up hook
function themename_theme_suggestions_page_alter(array &$suggestions, array $variables) {

  // Check if the content being loaded is a node
  if ($node = \Drupal::routeMatch()->getParameter('node')) {

    // Get the node's ID
    $nid = $node->id();

    // Get the URL alias based on the ID of the node
    $alias = \Drupal::service('path.alias_manager')->getAliasByPath('/node/'.$nid);

    // Check if the URL alias matches the styleguide's
    if ($alias == '/styleguide') {

      // Suggest Drupal to use whatever custom layout we have defined inside /templates
      $suggestions[] = 'page__styleguide';
    }
  }
}

I'm by no means a Drupal expert, quite the contrary, so I'm not entirely sure if this solution is properly executed or performant for that matter. If you know a better way to solve this, feel free to leave a comment!