So if you place any non-negligible value on future generations whose existence is threatened, reducing existential risk has to be the best possible contribution to humanity you are in a position to make.

This sentence smuggles in the assumption that we are in a position to reduce existential risk.

Two big risk are global warming and nuclear war.

The projections for large changes in climate depend on continuing growth in wealth and population in order to get the high levels of carbon dioxide emission needed to create the change. If it really goes horribly wrong, we are still looking at a self limiting problem with billions dying but billions living on in nuclear powered prosperity at higher latitudes. It is not an existential risk.

Nuclear war in the next hundred years is shaping up to be 2nd rank nations duking it out with 20kilo-ton fission weapons, not 200kilo-ton fusion weapons. That is enough to change building codes in ways that make current earthquake precautions seem cheap, but it is a long way short of an existential risk.

Existential risk might be large, but it comes from the unkown unkowns, not the known unkowns, and not even kowning that we don't know there is nothing useful we can do beyond maintaining a willingness to recognise a new danger if it makes its possibility known.