functions-custom-archive-taxonomy-route-with-template.php

<?php

/*
  This specific case creates a new route/URl to display Wordpress
  posts from a fixed category with a parameter-based taxonomy 
  term with a custom template.
  
  Needs template file: custom-template_category-tax_term.php
  Inside the template you can fetch the query-var with:
  $wp_query->get('my_tax_query_var')
  
  Change these to your likings:
  - my_new_route
  - my_tax_query_var
  - my_taxonomy_name
  - $query->set('cat', 1);
*/

/**
 * Create Rewrite-rule / Route
 * /my_new_route/[taxonomy_term]
 */
add_action('init', function() {
  add_rewrite_rule(
    '^my_new_route/([a-z0-9-]+)[/]?$',
    'index.php?my_tax_query_var=$matches[1]',
    'top'
  );
});


/**
 * Create new query var
 */
add_filter('query_vars', function ($query_vars){
  $query_vars[] = 'my_tax_query_var';
  return $query_vars;
});


/**
 * Use specific template when query_var 'my_tax_query_var' is set
 */
add_action('template_include', function($template) {
  if(get_query_var('my_tax_query_var') == false || get_query_var('my_tax_query_var') == '') {
    return $template;
  }

  // Check for given term, fail/404 if not found
  $tax_term = get_terms([
    'taxonomy' => 'my_taxonomy_name',
    'slug' => get_query_var('my_tax_query_var')
  ]);

  if(!$tax_term || empty($tax_term)) {
    status_header(404);
    get_template_part(404);
    exit();
  }

  return get_template_directory() . '/custom-template_category-tax_term.php';
});


/**
 * Modify the main query of the new template to only include 
 * the correct posts
 */
add_action('pre_get_posts', function($query) {
  if(!is_admin() && $query->is_main_query()) {
    if(get_query_var('my_tax_query_var') == false || get_query_var('my_tax_query_var') == '') {
      // do nothing...
    } else{
      // Set the fixed category
      $query->set('cat', 1);
      
      // Ignore stickies
      $query->set('ignore_sticky_posts', true);

      // Create topic_term tax query
      $tax_query_topic_term = [[
          'taxonomy' => 'my_taxonomy_name',
          'field' => 'slug',
          'terms' => get_query_var('my_tax_query_var')
        ]];
      $query->set('tax_query', $tax_query_topic_term);

      return;
    }
  }
});