If you set an absolute realism standard for hard SF, then what you're left with is a tiny rump of a subset of the stories by a tiny number of authors. There's barely any SF that meets that standard.
For me, I don't mind fantastical elements (FTL drives, wormholes, etc) as long as the rules they work by are consistent. Beyond that what makes SF 'Hard' for me is that they take real physics and explore it's consequences. That's the core of it.
For example, many of Larry Niven's stories are hard SF to me because he explores physical phenomena and their consequences. His characters encounter neutron stars and Neutronium objects, they encounter an anti-matter solar system, in one story there is a battle between buzzard ramjets which at the time he wrote it seemed like physically possible devices. OK so his characters often had to use magic FTL or other fantasy tech to get into those situations, but that doesn't change the fact that there is real science and (as far as he could get) plausible exploration of it in the books. That's all I ask for in hard SF.
As for the Expanse, they put in a lot of effort to show realistic zero gravity manoeuvring, fairly realistic space weaponry and habitats, including realistic effects of spin 'gravity'. Even the sociology was reasonably plausible. They picked a few specific things to 'break' physics for the purposes of enabling the rest of the stories to happen, and I'm ok with that.
