I personally disagree with the advice "all FTL is impossible so don't bother trying to be realistic or make use of science" heres why
not that I don't think that the fact that FTL is impossible is a completely valid and scientific viewpoint. by point is even if ftl is impossible in real life you can still:
- tie it into real mathematical theories
- involve speculative physic theories that although unproven and likely false can add legitimacy into your concept
- explain how it works and is interwind within our current understanding of science
- try and minimise the number of physics breaks within your system to only a few
I also think If you have been including real scientific concepts and ideas outside of the ftl system it would be more immersion breaking to suddenly stop including them more than an ftl system would
another reason I dislike the advice is that it assumes or sci fi readers have an understanding of relativity and causality. and know that their properties make functional ftl very unlikely. the average reader isn't going to read about a sci fi story using wormholes for travel using speculative physics to explain how it works (even if the nitty gritty details don't make sense) and another where the character travel with a magic flute that opens magical doorways and think "well since ftl breaks causality those are equally impossible"
despite this though I do agree strongly with the advice that the rules of how the ftl system works in universe "how fast it is, what resources it requires what it can and can't do etc are far more important than trying to make it feel scientificality plausible.