Yeah, Navidrome looks interesting. I use Plex for everything now (though it doesn't do books) but I kinda want to, as a project, acquire a used Intel 8th/later gen i3 or i5 workstation, throw Linux on it, and run my servers off of that, and just be able to remote in from my desktop. So if I did that, I'd probably run Jellyfin for movies/TV (with Plex as a backup until it's not viable... I feel like it won't be for long), Navidrome for music, and some others.

Yep, same here. Navidrome has been excellent, paired with Symphonium on mobile and Kavita has all the features I need. I mean audio books are just audio, if you have the metadata what's the difference?