I really liked The Deep a lot exactly as it was, but I definitely would not have objected to a meatier version of the book! Rivers Solomon is definitely capable of going meatier; their novel An Unkindness of Ghosts was brilliant, imo, and had that greater length to really dig into what it was doing.

For Deeplight, I've actually generally seen Frances Hardinge categorized as middle grade rather than YA, so that might explain why it didn't feel YA to you! YA seems to me to have codified itself into like....an identifiable set of tropes, these days, rather than just being "books intended for teens" like I feel the genre label ought to mean.