Audiobookshelf

Look, you've already got the best audiobook player. The problem you're having now is sourcing, the solution is to get a MyAnonymouse account. They do open registrations fairly often and it's literally the easiest private tracker to keep up with. You may not want to torrent, but I don't think you're gonna find a better way to get the selection you want. MAM really is the best.

If you want to host it, audiobook shelf is great. I use it for audiobooks, ebooks, and podcasts.

Other than that I'd just say get a library card and use Libby. If you have files in your phone playbook works but I don't remember if its on fdroid.

Voice has never made me want a different player, fwiw.

If you have a library card they may use the Libby app for ebooks and audiobooks. My apologies for a non-piracy answer.

Smart Audiobook Player

I use playbook. It's fine. Simple, good controls. Works with super-long files and smaller subset files.

I'm pretty sure I got it on fdroid.

Listen Audiobook Player

https://play.google.com/store/apps/details?id=com.acmeandroid.listen

I use Smart Audiobook Player. Only downside is renaming .m4b files to .m4a.