I have a separate library for audiobooks. I just create a book and download the metadata. I use the Publisher section to indicate where I bought it. I also edit the info section to include the narrator, which is useful as I have several versions of some books by different narrators. Initially, I was going to include cost, but since I already do that in my TBR spreadsheet, I didn't bother. I don't care about the cost once I've listened to it. I did this when I only had about 300 audiobooks (and now have 1,000 more than that). Don't make it too complicated - you just waste time when you could have been listening.
All my audio is sorted on a hard drive and backup hardrive - Genre, Author, series. Even though some of my authors write both scifi and fantasy, I just pick the one that best fits what I put them in. I don't buy it unless I can archive it somehow or unless it is very cheap with one time listen in mind (think Marie Kondo - $1.99 at Chirp).