Thank you page.

The custom menu is definitely the way to go. Post here if you run into issues!

As christogeretz said, try with the custom menu.

Also please note that if you don’t disable the canonical url,
your thank you page will be very easily found by searching.

So if you want to hide your thankyou page, it would be a good idea
to disable the canonical url.

To disable it, drop the line of code in your functions.php

remove_filter(‘template_redirect’, ‘redirect_canonical’);

or use the plugin.
https://www.ads-software.com/extend/plugins/disable-canonical-url-redirects/