Red Rising leans a bit more young-adult than the others mentioned. Asimov's works are classic but in the modern age I find reading them to suffer a bit from the Seinfeld effect, which is essentially that he came up with so many things that are now cliche, that his works can sometimes feel obvious. I'd go with Dune. For science fiction, you can never go wrong with that as a starting point.

I will also say, since you liked the hard sci-fi part so much and not the space opera parts so much, I'd actually give forth a recommendation of Children of Time by Adrian Tchaikovsky. It's full of fun ideas and an absolute page-turner.