Now, quibble if you like about Star Wars status as sci-fi or fantasy (I er on side of both because fuck you), but I like to think the whole idea of "Hard Sci-Fi = rocketpunk with super-duper compooters and other contemporary elements of futurism" just reveals a kind of jealousy for the soft sci-fi writers who can sit back and have fun with it, or the Golden Age writers who didn't have a half century's worth of space missions informing the readers expectations on the likelihood of ever seeing manned missions to other planets again. Really, take some of these Hard Sci-Fi settings that fit the stereotypical STL and rockets formula and ask yourself "does this story really have to take place in space? Does any one technology or scientific concept hold the plot together? Or is it just a retread of Golden Age sci-fi without the historical context that Asimov et. al. were writing in? Or worse, an attempt to one-up Star Trek by taking away the magic lightspeed engines and rubber forehead aliens?" It seems like a lot of people shoot their creativity in the foot by trying to satisfy only the criterion of plausibility-- and at that, only satisfying their own ideas of whats plausible while actually ignoring a lot of science and engineering realities that keep their futuristic technologies on the drawing board in real life. They should sit down first and ask themselves what exactly they want to write about, and whether or not it even needs the standard sci-fi setting, or whether it could simply happen today here on earth.Schmidt in an interview wrote:While, in one sense, I’m proud to keep Analog as hard science fiction, I also sometimes wish that term would go away. So many people use it in a way that they think is what I mean by it, and it’s completely different from what I really mean. Recently I got a story by somebody who said, ‘So and so said I should sell this to Analog because it’s full of clanking hardware.’ I have no intrinsic interest in clanking hardware. What I mean by ‘hard science fiction’ is actually pretty simple: there’s some element of speculative science or technology in it, which is so integral to the story that you can’t take it out without making the whole story collapse.
The second requirement is that there should be some attempt to make the science or technology speculation plausible. When I was teaching the science fiction course, one of the stories that I had people read was ‘Flowers for Algernon’ (one of my all-time favorites). It’s the quintessential example of meeting the requirement that there has to be some speculative element that you can’t take out without making the story collapse. Everything that’s important to Charlie comes out of that operation. And yet, because there are no rockets or robots and very little is said about the medical details, about half the students in my course were surprised that I even considered it science fiction. On the other hand, everybody thinks of Star Wars movies as science fiction, but they’re what my dad calls ‘really good Westerns with terrific special effects.
How does this relate to the thread at hand? Simple. What kind of technology is plausible for our hypothetical civilization? Answer: it depends on the perceptions of each poster who has advocated the Hard Sci-Fi civ's side thus far. That includes the OP and his mistaken belief that warp drives and wormholes are not true Hard Sci-Fi despite all the other things that get a pass in the hard sci-fi world, like spacecraft that can survive the stresses imposed by an STL relativistic drive. The instant you say "The only limits on its technological advancement are our current understanding of laws of physics and inherent engineering limitations," you have not described an interstellar civilization, or even an interplanetary one. You have described modern Earth, because we don't yet know if the kinds of civilizations Hard Sci-Fi likes to wank on about are actually possible within the realms of physics and engineering as we know it, let alone future history considerations like "will we avoid climatological disaster in the next fifty years?" or, "Will space colonization ever get off the ground or become appealing enough to society to actually happen?" All "Hard Sci-Fi" that involves space colonization makes implicit speculations about technological and social progress that cannot necessarily be taken for granted no matter how much time you give them to accomplish it in. So the answer to the OP's question is either "no one can know because the assumptions are subjective" or "None, because we don't even know if we can overcome all significant threats of our own extinction yet."
Now I would like to see someone write the story of how the speed of light was exceeded, and how that effects people. Not shoved into the backstory of a space opera, simply as a story of its own. THAT would be interesting science fiction.