Went with Thalia here in Germany (still less books than Audible) and used OoenAudible to convert all my books to .M4B-files and also all MP3-files from Thalia into the same file type as well.

Deployed an own instance of Audiobookshelf then and bought the plappa-app on iOS to stream or download the books, also having CarPlay-support here.

A little bit complicated but the result is worth it - and new books are bought on Thalia from now on. Works for me and my use case!