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.