

Audible + OpenAudible. OpenAudible does “stuff” and you end up with audio files, that you can listen on most devices. Don’t know and care how they do this. Its not free but so is Audible.
When you have an active Audible subscription, you also have access to free Audiobooks. You can download and convert them too. But be aware, that Audible is rate limited. Had downloaded a ton of free audiobooks and after a short limit (maybe 1 hour), I got a long limit for around 24 hours. But I still use Audible. I just have it as a backup and this way I can give my family access to the books I have. But so far my mother only listen to the ones I got for free. I like Science Fiction a lot but my mother not.
A college who I recommended Audiobookshelf, has a subscription from a German only site (Thalia), where apparently the Audiobooks can be downloaded as MP3s. So far I prefer Audible, even with DRM, just because the availability. Not all books I listen to, are available on that site or much later.
So far so good. The URL is correct, because its the external address. You also don’t need to publish both http and https ports. I only map external https to internal http but you can do https to https. No serious modern browser tries http first and because I always force https anyways, it doesn’t need to be public. Only the reverse proxy may need it, for Let’s Encrypt.
Both UDP aren’t needed for public access. I only have mapped 8096 to my reverse proxy and it works.