Hey, thanks for the Audiobookshelf recommendation!
Recently I was looking for a audiobook player and Smart Audiobook Player was the best I found, but Audiobookshelf have almost all of the functions I liked like simultaneous dual timeline bar (one for the whole book and another for the current chapter), auto sleep, shake to reset sleep and so on. Just missed the small beep alert before the audio fade out on sleep.
Anyway, and all of this with the advantage of using my already selfhosted data, so no need to keep transferring files manually to the device. And all the progress are synced back to my homeserver, so I can use multiple devices without any problems.