There is absolutely no reason to think that radiation damage should be linear. In fact, we can objectively say that isn’t the case because we KNOW about damage correcting pathways. The linear model, used for public safety limits, is picked as a conservative method because we don’t have a better model (and collecting data to make a better model would be unethical).

The “small dose radiation is actually beneficial” part is pure conjecture, and almost certainly wrong. But unrelated to the issue of the validity of the linear model.