Hello,
01. You used Gutenberg for the Checkout page. In order to disable it, edit this page and remove all Gutenberg blocks. Then define such a shortcode for the checkout: https://take.ms/8yG9B and test how it works.
02. Navigate to WooCommerce -> Setting -> Products -> Enable AJAX add to cart buttons on archives.
https://prnt.sc/7WwjYQEdcMh8
You can configure the option in the WoodMart > Header Builder > Edit Current Header configure cart settings.
Best Regards,