Sorry, but Accelerando is horrible. I recommend Singularity Sky, the weird one, not watered down for the mainstream. The characters are flat, but the weirdness on route, is captured much better.

I have a soft spot for Accelerando because it was my first encounter with the exo-cortex (or perhaps more accurately called cybernetics) concept, aka computers as an external brain. Also it was my first encounter with the concept of smart contracts, laws & corporations run purely as code.

I just read Diaspora, and find it far from hard sci-fi.

The Wikipedia page says “Egan invents several new theories of physics”, and that to me is not hard sci-fi.

I think the best hard sci-fi can include cutting edge theories or even ideas that the author themselves are proposing, based on a reasonable understanding of the existing science.

At the time many of Greg Egan's early books from the 90s/00s were written, there were a lot more competing ideas or theories in quantum mechanics compared to now. In his books he basically explores some of those ideas, and pushes the boundaries.

The best sci-fi to me inspire future scientists or engineers to explore novel approaches.