What was your time frame like from completing the book to penning the review?

I took a while. And as you said, the book kind of fermented and lingered in my mind, so it got worse at some stages, but more manageable at the same time.

I found the book's "lessons" and themes growing on me

Yes, that is exactly what it is. The more it lingers, the more you get from it. And I am going out on a limb here, but that is what distinguishes good books from bad books for me. If it lingers on after the read, it is a fantastic book.

a journey through humanity

I am not sure, but I guess this was it. And it really achieved it. I think I have read 3 or so of his other books, and each one of them is definitely a journey through humanity, quite literal at times!