1) Go to Appearance > Themes > install and activate Storefront theme for a few mins.
2) Check if you have the problem with thank you page when the default theme is enabled.
3) Let me know about results.
It won’t solve the issue but we will know if the problem is related to our theme files or to WooCommerce plugin. We should find out what causes the issue.
If problem appears with our theme only and related to theme files we’ll be able to fix it. But if it does not work even with official WC theme then you need to contact WC plugin support, we won’t be able to fix this issue.