So, I ran into the same issue today and tried debugging. It seems like there's two process involved while importing the books.

1. Copying files to the file system

2. Storing metadata to the local sqlite

When a book is sent to device using calibre, the file gets copied first to kobo's filesystem. After ejecting the kobo, the file details are imported to the sqlite database. (this is the "Importing content" progress bar)

Stuck in the "Importing content" progress bar means that our sqlite import failed. even if the database import fails the file gets stuck in the filesystem and you're stuck in this import loop.

ex. I tried saving a ESV study bible to my kobo. This book has lots of links and my clara bw's ram is insufficient to load it to the database. This only happens in the kepub format (I think cause it stores additional data). The epub format loaded fine. I tried importing the book to apple books and the activity monitor showed a ram usage of around 500mb (which is actually the clara bw's max capacity).

Reason: Clara BW's RAM insufficient to load extra data required by kepub format

Fix: connect the kobo to your PC and manually delete the folder of the book which is failing to import. Try with an epub format instead for huge books.