Relativistic starships are impossible because they require impossible amounts of fuel. "If you can create a ship that is capable of 1g acceleration" is doing a lot of heavy lifting. The rocket equation means you need to take along exponentially increasing amounts of fuel

Even antimatter rockets top out at 50% of light speed. Laser boost like with Dyson Swarm could get similar speeds because time dilation slows down the acceleration.

This isn’t gonna work, space isn’t truly empty. Even with antimatter propulsion the interstellar medium will start to vaporize your ship at speed above 0.2c.

> If you can create a ship that is capable of 1g acceleration, it doesn't just stop accelerating as it reaches the speed of light

For any object with nonzero rest mass, reaching exactly the speed of light in vacuum would require infinite energy.