Check out the iOS app Bound if you ever have a bunch of downloaded audiobook files that can’t fit in Audible, Libby, etc. I’m using it with the MP3s for Dragonsteel Prime and it works great.

https://apps.apple.com/us/app/bound-audiobook-player/id1041727137 https://www.dragonsteelbooks.com/products/dragonsteel-prime-audiobook