> We know astronauts to Mars will be in space for nearly 2 years. We should have had him stay 2 years.

However they will likely be in microgravity for 4-5 months per leg of the trip. During the time at Mars they will experience gravity (albeit less). Plus the radiation flux on the Martian surface is only slightly higher than on the ISS. The thin Martian atmosphere alone blocks radiation surprisingly well.

New classes of rockets like Starship from SpaceX pack enough delta_v to lower the old numbers (6-9 months) significantly.

Theoretically Starship could reach Mars in about 80 days with 100 tons of payload and fully refilled in LEO. But then the arrival velocity at the Martian atmosphere would be too high for the heat shield. So the trip duration has a lower limit of roughly 4 months in a good synod.