I keep holding my breath for a linux client, but so far no dice.
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.
I forgot where I saw this (a tweet?) but iCloud Drive may have moved in Catalina. It should only be a matter of pointing SyncThing to the new location, but it’s something to be aware of to keep your solution working.