Indeed it is simpler. I have 18Gb space on DropBox but my library exceeds that storage. Also I now understand that, in order for the webdav access to work, my MacBook would have to be running. When the MacBook is off/sleeping webdav won't work. So I have to think again.
In the short term I use this to easily send books from my Mac or Phone:
https://send.djazz.se It works very well. Longer term I'll have to either rotate books on DropBox, buy a Kobo with more than 8Gb storage or something else.