normally you need a single question (not a single workflow) for each folder level you would like to browse, that's the typical way of configuring sub levels for any type of connector (Sharepoint but also any other which has a folder structure). The first question is pointing for the first level you want to browse and every additional question will take as parent the variable of the previous question so that they will browse each other sub level, till the level you would like to reach.
In order to use one unique question we have the new "Selected with tree" question type which will start from a specified level and display a tree in the client in order to browse completely the objects in one unique question.
The new type is available only for Web clients and since 4.11, so you rather update to the latest version available (06.20 for both v4 and v5). Currently this type of question is only available for certain connectors because not all of them implemented it yet: Windows File System, Google Drive, OneDrive (both personal and for business), Sharepoint (both on premise and online).