I use m4b-tool[1], tone[2] and audiobookshelf[3] together with an LG G5 H850 smartphone[8] with Bang & Olufsen Hifi-Plus Module for Audio Only and I am pretty happy with this config. For Music I use Navidrome[5] and Substreamer App[7]. Maybe I'll try out Jellyfin[4] or maybe Plex[6], but I really don't wanna go closed source.
I also thought about writing something self hosted in C# to have ONE solution for audiobooks, podcasts and music and started a small private project, but this will take a while until it is ready to release something...
You may ask: Why an LG G5 H850? Well, its relatively small and cheap (about 50 - 80 bucks used) it has an audio Jack, USB-C, you can change the battery, it can hold up to 2TB microSD storage, has an HiFi Plus module for audio enthusiasts and a descent screen. Besides that it can run lineage os...
Note: I'm the author of the first two projects :-)
[1]: https://github.com/sandreas/m4b-tool
[2]: https://github.com/sandreas/tone
[3]: https://github.com/advplyr/audiobookshelf
[5]: https://www.navidrome.org/
[6]: https://www.plex.tv