Edit parent or base slug page with a page builder

Closest thing I can think of that would actually work is having “case-studies” be the page you choose for this page builder based spot, and then anything else besides “case-studies” be the post type you choose, or at least not the default-used “rewrite slug” for the post type.

I say that last part because having a page named “case-studies” and a post type that has single posts using “case-studies” in its permalinks will likely cause WordPress to not know what to query for or query for the wrong thing in many cases. That’s because the single post url for “/case-studies/my-case/” would get considered a child page of the first part above, and not a single in a post type. These are just possible suggestions, but perhaps “case-study” or “case” for the post type permalinks would potentially fit your needs.