I was thinking about this recently while reading the fourth book in the series and you put my thoughts into words tidily.

Tangentially, I also think the time gap between the third and fourth books (some 3,500 years) necessitated some more creativity with changes to the culture but I got the impression that the author still reused a lot of the same references despite the gap which felt kind of lazy, like humanity was just TOO stagnant for that long. That's when I started thinking that it must be a difficult endeavor to just invent a lot of new culture, especially when you've got to do service to the previous books in the series. The fifth book appears to follow the same trend despite another 1,500 years of time.