Can you clarify what customization you did on the theme? I don’t see your site having an
x-child-theme-active, I believe the customization that you’re talking about is not a template level customization, or else you’re risking to lose all your customization when you update the theme.
However, if the customization you’re talking about is the page design you did on Cornerstone, having PRO is actually a good upgrade.
Please clarify first, what customization you’re talking about, and we will provide a recommendation base on that.
Please note, that we encourage the user to have always the latest version of Wordpress, Theme, and Plugins.
And another important thing, which might be the real culprit why the site does not load after the update.
Remember to clear all your caching features (plugin, server-side Optimizer, CDN (CLOUDFLARE), and browser’s cache) after updating so that the code from the latest release is always in use. This will help you to avoid any potential errors.
Let us know how it goes,