In @nerdias comment in this thread about setting up plex for audiobooks, there is also a section with a link about de-drm’ing your audible books.
I havent troed it out myself, just stumbled upon it while checking out the thread, but I thought I’d point it out.
So OpenAudible kinda gives me the information I need in the metadata, so I ended up making a simple python script that copies them around based on that.