Then you need to provide more details. I have used many versions of macOS with Kindles including 12.4 and attaching a Kindle has always shown it as an external drive (called Kindle)
What do you mean by the device itself - how does a device show up on macOS?
For the Kobo Libre, I can indeed see a mounted volume, but copying a file into it does not consistently make that book available for reading.
Given I don't have these problems when I use Calibre, I'd definitely say it makes my life easier.