Its really not hard! I am with GP in that most scifi with FTL comes off as silly, perhaps fun and well written but not to be taken seriously in the sense of its worldbuilding being immersive or intriguing.
Almost all my favorite books almost exclusively occupy settings with laws of physics that are internally consistent and believable and therefore have no FTL. See "The Expanse", "Three Body", "Blindsight", "The Sunflower Cycle", "Rendezvous with Rama" and too many others to name.
Mild spoilers for The Expanse:
It does have FTL travel, but it is presented as alien and has a limited impact on the day-to-day lives of characters. I get the feeling this was done for narrative reasons primarily, but it helps with "sci-fi hardness".
Generally I think the impact of instant communication and near-instant travel (or lack thereof) is not talked about enough. Most sci-fi stories derive from the present day, and keep this aspect of today (as do many fantasy stories, interestingly), but it's a narrative choice with large impacts.
Yeah I guess it has FTL but its not really FTL in the same way I think GP meant. It could just as easily be simulation or something else. Its not warp or hyperdrive and it doesnt really violate the internal consistency of the world building since its totally alien clarketech
Does it? The sophons are capped at light speed though they communicate FTL. The whole curvature propulsion I thought had the ships moving at light speed, not exceeding it. Three Body has plenty of clarketech but most of the drama comes from it being grounded in something kind of like actual the laws of physics.
Almost all my favorite books almost exclusively occupy settings with laws of physics that are internally consistent and believable and therefore have no FTL. See "The Expanse", "Three Body", "Blindsight", "The Sunflower Cycle", "Rendezvous with Rama" and too many others to name.