Guess it's time for the lore to kick in once more.
Apparently the Supercruise travel is an idealized form of Alcubierre Drive, with all the good things and none of the bad things
In Supercruise we dont travel faster than the light of speed so don't get anywhere near relativistic time effects.
The FSD is compressing the space around our ship and we travel in that bubble of compressed space at a fixed speed, probably around 100m/s (yea, remember that even a non-engineered T9 can get into supercruise

).
How big is the compression level depends of nearby gravitational wells.
So what we see as 30c does not means that we actually travel with a speed 30 times bigger than the speed of light.
That is only the apparent speed, set up in a reference system we can relate to.
If the ship would be telling you that we move with 100m/s in a compression bubble of 90 millions to 1 it could get confusing.