redirect after checkout

I don’t think there is an easier way.

Start by making a child theme:
http://codex.wordpress.org/Child_Themes

Then make a plain text file called functions.php in the root of your child theme. It should contain:

<?php
  add_action ('woocommerce_thankyou', 'newthankyou');
  function newthankyou($order_id) {
    $order = new WC_Order($order_id);
    foreach($order->get_items() as $item) {
      if ($item['product_id'] == 154) {
        wp_redirect('http://www.yoururl.com/your-thank-you-page');
      }
    }
  }

Alter the product id and the url to suit your needs.