Syncing audiobooks from Mac to iPhone

Hi, so I have a library of audio books on my Mac. I have decided to change to Apple Books, I have imported my library into the app on my Mac. Now I want to send them to my iPhone, I read up about going through the finder, selecting my iPhone and then on the button bar go to audiobooks, now the problem is I can see my iPhone in finder but there is no “audiobooks” button. I can see movies tv etc etc. why is this so hard to get it synced? I could sync them via Apple Music but I ideally want to have everything separated out and now be using just Apple apps for podcasts, music and audiobooks.

side note - every other app syncs fine including music.