if only the domain changes, you could try using relative urls:
document.location="/thank-you";
But if the domain you’re redirecting to is different than the page you’re on, a simple way to fix this problem would be to add a customizer value redirect_domain, and then make it a global variable in footer.php
<script>var RedirectUrl=<?php echo get_theme_mod(redirect_domain);?></script>
then in your on_sent_ok you should be able to use the RedirectUrl variable