I'm not sure I completely agree with your last assertion (except according to a very rigorous definition of "optimize"). While people do very much differ, there are certain things that predictably make the majority of people happier. Social connectedness, for example. We may not be able to truly optimize for these things, but I think we can reliably improve human wellbeing at scale. A successful example from the past would be the efforts to add more green spaces to cities. People like parks, and they're happier on average when they have access to them.