Two more attempts as I improve this aspect of my game. Using the Dutch, I was cruising along happily with Pacifism, until I hit Ideologies. I chose Order, the world Ideology, but still found myself cratering at -67.
I did a bit of Maoist self-criticism, and saw that I could have built Med Labs earlier, as well as stayed closer to the AI culturally. So I ted again with Carthage, and was cruising along with Pacifism again, no lower than 52 happy after choosing Freedom (World Ideology). My happiness eventually fell quickly to around 20, probably due to pressure from my two Autocrat neighbors. When one of them DoW'd me, I dropped briefly to -5. When I lost a city, I fell 53 points to -50.
That seems weird, but all in all, my happiness is improving, although I'm banking an awful lot on the successful spread of my religion with Pacifism.
Any comments on how to maintain late-game happiness, and what can make it collapse?