John has it nailed. If you put in a RECOMPILE hint, nothing gets cached because there's no need since each execution gets a new compilation.

Why would Optimize for Ad Hoc not be applicable? Except for discussion around the RECOMPILE hint & caching, it sounds very applicable. You seem to be building a very ad hoc focused process here.

Also, exec does not allow for parameterized queries which introduces the possibility of SQL Injection. Be sure you're avoiding that issue.