I read them all at once in a single volume. I agree, they become a slog in the middle and hard to even differentiate; I guess it helps relate to the state of mind of the protagonist🤷♀️. I remember really liking the end of the 4th(?) book and thinking it was the perfect finish.
Right after, I read the Borne series and Veniss Underground (most beautiful/gross scene I’ve ever read in English in this one) and found them incredible and way easier to get into.