Taxes are charged on a state/local basis, so it'll vary by area. I pay around $5.50/mo in tax+fees (Consumer Cellular, IL).
Fees, don't know, not a Sprint customer, and I don't really see any clear indication on the site.
I don't know how much data Discord eats. Depends on whether you are in a room with audio or not. Probably minimal if text-only, more if audio is involved.
I would think it would be fine for audiobooks; I bought a few MP3s from Amazon a few years back, and they normally encoded with a variable-rate MP3 that ran somewhere between 200-300 Kbps.
Other caveat: You also have to factor in the value of a hard pull on your credit report if you are planning on applying for a mortgage or any other credit in the near future.
For someone who hates phones, as I do, it seems to be a good deal. Half the price of what I'm paying with CC, and that's already a pretty good price. CC does have very good customer service, though, so I would be giving that up.