This is already an unrealistic thought experiment, since continuous 1g acceleration for 20 years is so ridiculous that it requires a magic engine (i.e. consumin

This is already an unrealistic thought experiment, since continuous 1g acceleration for 20 years is so ridiculous that it requires a magic engine (i.e. consuming most of the ship mass in a matter-antimatter reaction) so if we're at it, we can also assume magic shielding which will also shield the crew from all that antimatter and its annihilation. It's not a coincidence that the original post relativistic spaceship doesn't even bother considering larger accelerations than 0.1g, since achieving even that is a wild assumption.