There's probably a real business opportunity in selling a product that abstracts this plumbing into an SDK for developers, so they can give their users a polished flow for authenticating and configuring the storage on the frontend, and then interact with the user's chosen directory on the backend. I imagine it's been done before with various "file upload" products but I'm not familiar with the space.

As a user, I would love this kind of integration because it means data portability is built into the product.

Sorry, it seems my comment was misleading. There is no integration with sync apps.

My app is a Mac app, so it just reads/writes JSON files to disk in a folder that the user selects. The user selects a folder in iCloud drive, Dropbox, etc.

There is nothing special about my code, except that it is written with the expectation that some service modifies the files on disk in the background. Every JSON file has a uuid, so I can track renames of the files. But for the most part it's just a lot of handwritten code that converts JSON into objects and vice versa.