Audiobooks are usually mono, so 700 kbit/s for uncompressed CD quality. To me parent's numbers sounds like uncompressed. I suspect something went wrong with de-drm/decoding.
Should be more like 48 kbit/s for basically transparent mono voice with opus. At this rate, 9 solid months will fit into 142 GB...
Hmm... well, I do tend to try to maximize the amount of listening I get per credit, so I have lots of 20+ hour books. Stephen Fry's reading of the complete works of Sherlock Holmes is nearly 63 hours [1] and that's just a "single" audiobook (and highly recommended). Lots of Great Courses recordings are similar. Rise and Fall of the Third Reich is 57 hours, etc. Also, there's a ton of metadata like images and PDFs, though I'm not sure how much that takes up. And in fact (just checking again) I'm at only 638 books, so it was probably more like 625 books when I did this. Apparently it adds up.
id say change the audio format and cut the bit-rate roughly in half.
but welcome to data hoarding, start with a 8tb external wd dirve!