you would never build a spaceship to accelerate to the speed of light. For the reasons you stated AND the fact that it would require massive amount of energy.
But you could warp space/time by using wormholes and arrive at your destination immediately.
It appears aliens have discovered how to do that. That is how they achieve instantaneous 90 degree turns. They are not setting things on fire and squirting it out the back, they are using some physics we don’t yet understand (here on earth) to jump to new locations
I liked ‘The Rendlesham Enigma’book.Answered some questions I had.