We don't actually need Starship to be man-rated, though. Use it to launch cargo, fuel, and unmanned vehicles, and then send astronauts up on a Falcon 9 (which is already routinely shuttling people to space) and have them transfer over in orbit.
We don't even need Starship to be reusable for it to be cheaper than SLS. Though reusability is another whole order of magnitude or two of improvement.