
First contact isn't all fun and games. Ariel Blum is pushing thirty and doesn't have much to show for it. His computer programming skills are producing nothing but pony-themed video games for little girls. His love life is a slow-motion train wreck, and whenever he tries to make something of his life, he finds himself back on the couch, replaying the games of his youth. Then the aliens show up. Out of the sky comes the Constellation: a swarm of anarchist anthropologists, exploring our seas, cataloguing our plants, editing our wikis, and eating our Twinkies. No one knows how to respond--except for nerds like Ariel who've been reading, role-playing and wargaming first-contact scenarios their entire lives. Ariel sees the aliens' computers, and he knows that wherever there are computers, there are video games. Ariel just wants to start a business translating alien games so they can be played on human computers. But a simple cultural exchange turns up ancient secrets, government conspiracies, and unconventional anthropology techniques that threaten humanity as we know it. If Ariel wants his species to have a future, he's going to have to take the step that nothing on Earth could make him take. He'll have to grow up.
"Constellation Games" by Leonard Richardson (also known for the Beautiful Soup Python library!) https://constellation.crummy.com/
"Happy Snak" by Nicole Kimberling https://www.nicolekimberling.com/happy-snak
Alistair Reynolds has a number of good books which can capture some of the same darkness you'd find in part's of Banks's work. The Revelation Space books are great in their gothic way, and House of Suns has a lot of the galactic wonder aspects of the Culture books.
Ken MacLeod as a friend of Bank's and a good writer. His Fall Revolution books are probably the best known and explore various radical leftish ideas for organizing society across the nearish future.
And this last is a bit of an oddball but Leonard Richardson's Constellation Games was a book I really enjoyed that starts with aliens coming to Earth looking to help out and our protagonist, a video game reviewer, asking for alien games to review but ends up in a place that resonated with The Culture very strongly.
Anything by Greg Egan is probably going to be the hardest sci-fi you've ever read. Dude wrote a book where he considered the ramifications of a universe built on a positive-definite Riemannian metric, and another one where the universe has 2 time dimensions.
Robert L. Forward's Dragon's Egg explores what life might look like if it evolved on a neutron star.
Adrian Tchaikovsky's Children of Time and Children of Ruin explore the evolution of other earth species if they were given a kick towards sapience.
Vernor Vinge's A Fire Upon the Deep is half space adventure (not very hard) and half an exploration of a lifeform which only achieves sapience in small groups. A Deepness in the Sky is generally harder and explores a lot of things, including the power of focused human attention, the difficulty of galactic scale civilization, and alien life evolved in a star system where the star periodically dims.
Steven Baxter and Clarke collaborated on The Light of Other Days, which explores the technical and sociological consequences of a device which allows you to see the past.
With a broad interpretation of 'hard' I can highly recommend Ursula Le Guin's The Left Hand of Darkness and The Dispossessed, which have soft science but hard humanity.
Similarly The Long Earth series, a collaboration between Pratchett and Baxter, where it seems Baxter handles the sociological and technological consequences of the science while Pratchett handles the characters and philosophy.
Asimov's I, Robot is an exploration of what happens when you try to constrain intelligence with rules.
Asimov's Foundation Trilogy gets a lot of hype, but it isn't very hard and I also found it utterly mediocre. Instead I recommend The God's Themselves, which is so good it's like Asimov was channeling a much better writer to get his ideas down. It explores the limited interaction of our universe with one that has slightly different physical properties.
Also perhaps stretching the definition of 'hard', but I want to recommend it because it's relatively unknown, is Leonard Richardson's Constellation Games, in which an incredibly advanced multi-species anarchic alien civilization makes first contact with humanity, and the protagonist really just wants to play their video games. It's actually harder sci-fi than it sounds.
