That's a good point. I have no idea actually where the practical limits would be, especially considering options that arise if the ship has access to a lot of electrical power to generate a protective magnetic field. But even at 0.3c, the trip to Proxima could still be worthwhile for an unmodified human.
You need to use the same amount of energy and time to decelerate, as you did to accelerate.
So you'd only be able to sustain constant acceleration for half the distance. Then deceleration for the second half of the journey.
That would make the travel time much, much longer than your calculations, since you'd be traveling at relatively slow speed for both the first & last 20-25% of the distance.
