Mass is all it takes for protection from radiation. Sand bags will do short term, a terraformed atmosphere will do long term. Whethor or not additional measures will be needed depends on the amount of volatiles still present on Mars and how massive the terraformed atmosphere would be. It is quite possible it will contain 2.5 times as much mass per unit area as Earth's which provides a significant additional protection to accomodate the lack of magnetosphere.

You have generalized a huge issue for space travel into a big one on the surface of worlds where protection is readily available. Mass is not a simple solution during transits because it is expensive to launch mass to the Moon or Mars. It is not prohibitive to launch empty sandbags to be filled on arrival.

Regards,
Mark