There is, to a first approximation, nothing you can do about this; And it is in fact up there around our thresholds for dose limit, unless you're satisfied with increasing the mission mass by a factor of a thousand. The only practical thing to do is to adjust our thresholds.
But screw it - we're talking about landing on another planet, about millions of man-years of time investment, about activities plenty of people would donate DALYs to accomplish; About activities they will usually have to donate their entire career and their marriages to accomplish anyway, where the unknown unknown risks of sudden death are expected to be of a higher magnitude in the first place. Thresholds are silly in that context.
For comparison:
Flight attendants who have been on the job between five years and their whole careers have ~600% of the breast cancer (and likely other cancer) risk of the median person.
Regular smokers have 2600% the risk of lung cancer as people who have never smoked.
Pay a smoker a million dollars to sign a contract stating they will never smoke again, send him to Mars, then pay a private investigator to follow him around for the rest of his life ensuring he does not start smoking, and he will have a vastly lower rate of cancer at vastly lower monetary outlay than if he had been rejected at the first astronaut screening and NASA had attempted to mitigate radiation risk from GCRs.