Personally, I don’t think Plex is great for audiobooks. I prefer audiobookshelf. It’s FOSS and brilliant for audiobooks.

I used to use Booksonic, and it worked pretty well. I’ve since switched to Audiobookshelf, and it’s been great. Client/server works pretty smoothly, and I haven’t really had any problems with it.

As far as I am aware, Chronicle Audiobook Player is the only android one that works with Plex.

It does the trick, but I wouldn’t consider it a particularly outstanding audiobook player in general. I use Smart Audiobooks for non-Plex ones but Chronicle to play directly from my Plex server.