folder in the way conflict/error when using /move API endpoint

It looks like you already got this working.

For reference, the /move endpoint lets you move items, be it files or folders. Both the to_path and from_path must specify the full, exact path of the existing item and the new desired path, respectively. If you wanted to move an entire folder, which is currently at /folder1, to be a folder at named folder2.1, inside a folder at /folder2, you need to make sure there isn't already a folder at /folder2/folder2.1 (even if the existing folder2.1 is empty).

Alternatively, to move all of the contents of a folder from one folder to another, you have to loop through to move each item as desired, like you described in your answer.