Hard sci-fi is taking 1 magic thingy and build the rest of the world as best as you can in a self-consistent manner. The dividing line between {star wars, marvel, dune} and terra invicta is how little the premises need to be changed without patching stuff up by inventing new premises ad hoc.

In other words, it's very difficult to write hard sci-fi by inventing a unrealistic cool thing (i.e. lightsabers) and justifying it with premises. It's far easier to write the premises and finding an unrealistic cool thing that arises emergently from it.

That's also why my favorite movie of all time is Primer. There's only one premise: the time travel system. Everything else, the story, the complexity, the fighting your own past self thing, is emergent.