Sync failed again earlier today (after a couple of days working correctly, I'm really
about why a setting that didn't give any issue for a couple of days started to give me issues today), draining my poor battery from 90 to 55 in a couple of hours.
I deleted:
Books
Fandom 1
collections on the device, and the sync finished (and - as expected - the drainage stopped), I suspect the faulty one was "Books".
I'm going to take a look at the sqlite file next time I connect my Kobo to check if I see something glaring wrong there