So exactly why do you need access to the file system? You have access to a location where you can share files between apps. Any application that supports access to file picker gives you access to any of your installed document providers like iCloud, Google Drive, Box, an attached USB drive, a network drive and the local device.

And direct access via the Files app.

The one thing missing is that admittedly there is no way to add your own music to the music library on the phone itself.