a. Audiobookshelf: New, still being actively developed. Android client though in beta is quite decent. iOS client still needs work. Multi-Platform (https://www.audiobookshelf.org/)
b. Plex with Prologue (https://prologue.audio/) (iOS)/Bookcamp (https://www.bookcamp.app/) (iOS/Android)/Symphonium (https://symfonium.app/) (Android): In terms of ease of use and features, Prologue beats all the other apps however the other apps mentioned provide a good experience
My preference of the above is LISTEN AUDIOBOOK PLAYER. I, too, think it has a better interface than SABP. Has as many if not more features. And if you have downloaded Libby or Overdrive mp3 files that can be played in third party players, the LISTEN can decode/display/use the "chapters" in those mp3 files. So can the Sirin Audiobook Player (Android) and Bound (iOS). SmartAudiobook Player cannot display "chapters" for these mp3 files, only "parts" or "files", which do not correspond to actual chapters.