The Expanse (both the novels and the TV show) are fairly “hard” sci-fi for the most part, but are also a space opera mixed with somewhat Game of Thrones-style politics. There’s a couple of things you’d have to look past - the Epstein drives that allow reasonably quick travel throughout the solar system are impossibly efficient, for one, and the show cuts corners with the science now and then to speed things up and keep the show on budget. But the nature of the existential threat (comparable to Game of Thrones’ White Walkers in a way) might turn you off, as it violates the laws of physics, however the characters discovering that it seems to ignore physics as we understand it is a huge plot point, so I’m not sure what your feelings on that will be.
It does fit the “space warfare” bit that you’re looking for pretty well - crash couches and a drug cocktail called “juice” that allows passengers to survive high-G maneuvers, fighting that takes place over long distances with rail guns that rip through the entire hull of the ship including the depressurization that would occur (without the “sucking everything out of the room like a vacuum” trope), near-combat guns that are computer/AI controlled rather than manned by a human gunner, etc.
Give it a shot. You may enjoy the books (Leviathan Wakes being the first) more than the TV show, just because, like I said, the show has to compromise now and then with the realism/hard sci-fi aspect (one particular example of orbital mechanics depicted on the show is pretty glaring), and after the first book the existential threat I mentioned has more and more of an effect on the story, which kind of kills your “diamond-hard science fiction” request depending on your feelings on how (or if) non-terrestrial entities can be depicted in hard sci-fi, but it’s worth a shot.
(I tried to be as vague and spoiler-free as I could when talking about the “existential threat” in the series, but I had to at least bring it up just in case that kills the “hard sci-fi” part for you.)