So niceness has individual hedonic benefits but results in fewer offspring?

That seems plausible to me. I find it easy to imagine that being nicer than the reproductive optimum could make one happier, even in the ancestral environment. Of course, there are stupid ways to be nice that wouldn't make one happier. And I'm sure that one can be too nice. But why would we expect that the hedonic and reproductive optima were ever the same?

Niceness can also have individual benefits, if you need other people's voluntary cooperation. But in our society the role of voluntary cooperation is getting smaller, because it is replaced by institutions and financial transactions. Also the strangers you interact with online are unlikely to provide you any specific help in real life. (Speaking in general, because in your specific case their donations are welcome.)

Furthermore, in a small society where everyone knows everyone, your niceness or rudeness might also influence how people treat your relatives. Which in turn would provide your relatives an incentive to fix your behavior.