Cloud File Storage with App Engine AND Web access for superusers

We are running an in house data collection app (currently only android) and need to set up a better file based cloud storage with app engine that can also be accessed by superusers outside the app eg web browser, to manage files and file structure.

We are currently utilising OneDrive personal for file storage but this requires login/authorisation by the user at runtime. With around 50 users per onedrive account (not good) this means handing out the login (even worse). With so many user per account MS security sometimes asks for an additional security code to login (really bad!).

Firebase storage looks like a good file storage option but we also need superuser access to the file system eg like OneDrive front end. Do we need and is it even possible to create a web interface to the same bucket accessed by the app for firebase?

Should we consider FTP?!?

So what options are there for cloud storage app engine that doesn't require runtime user login/authorisation but with a backend file manager like access for superusers to manage/upload/download files.

Asking too much?