macOS offers us a super simple way to fully encrypt an APFS formatted external drive: Right-click on the volume in the Finder and choose the option "Encrypt". Carbon Copy Cloner (CCC) also recommends this in its KB article Enabling encryption on a volume that will not contain an installation of macOS.
But macOS is a bit too transparent on that task. It simply starts encrypting the volume without any feedback or progress bar (WTF?). Even Disk Utility just shows the volume as APFS (Encrypted) - as it would have already completed full volume encryption.
The fdesetup utility (FileVault configuration tool) gives us more details, e.g.:
$ fdesetup status -device /Volumes/MY_DRIVE -extend
FileVault is On.
Volume is APFS. (FileVault Enabled)
Encryption in progress: Percent completed = 11. Estimated completion in (Calculating)
Encryption in progress: Percent completed = 47. Estimated completion in 5 hours 5 minutes
In case you won't specify -device, it will just display the FileVault encryption status of your system volume. On APFS volumes, the -extended option will give continuous updates and estimated completion times during encryption and decryption phases.
Exactly what I was looking for!
... and in case you have plugged the device into the wrong MacBook, the one with which you need to run out of house and hop on your bike right now: No worries, simply unmount the external drive. Full disk encryption will continue next time you plug it in again.
