I did a ghetto solution that syncs my iCloud Drive to my linux box:

1- Install SyncThing on both your Mac and Linux box.

2- Point Mac client to ~/Library/Mobile Documents/com~apple~CloudDocs

3- Optional, for extra privacy run your own discovery/relay server.

4- Enjoy iCloud Drive on Linux.

You could push it much further by running the native app in a Windows VM and sync more than just your files.