I see all of your licenses are Pro. If in the future, you're allowed to use Cornerstone, you'd need to purchase an X or a Standalone Cornerstone license so you'll be able to use Cornerstone for all the content that was built with Pro Content Builder.
If what you mean is just use other page builders but you still use the Pro theme, you need to manually create the pages with the other page builders and after that, you can disable the Content Builder in Pro > Settings > Permissions.
Regarding child theme customizations, you should be able to move all your none theme related functions to another child theme. In case you'll use X, follow the Updating Your Child Theme instruction at https://theme.co/apex/forum/t/x-to-pro-conversion/104
Hope that helps.