You can try and use the WC session:
Documentation URL:
https://docs.woocommerce.com/wc-apidocs/class-WC_Session.html
StackOverflow discussion.
https://stackoverflow.com/questions/27199656/how-to-use-session-in-woocommerce-pages-like-we-use-normal-php-session
And once you get to the Thank you page you’ll destroy it.
Haven’t tried it yet by coding it but that should work.