But the SIAI is asking for the better part of your income and resources.

If you are a hard-core consequentialist altruist who doesn't balance against other less impartial desires you'll wind up doing that eventually for something. Peter Singer's "Famine, Affluence, and Morality" is decades old, and there's still a lot of suffering to relieve. Not to mention the Nuclear Threat Initiative, or funding research into DNA vaccines, or political lobbying, etc. The question of how much you're willing to sacrifice in exchange for helping various numbers of people or influencing extinction risks in various ways is separate from data about the various options. No one is forcing you to reduce existential risk (except insofar as tax dollars go to doing so), certainly not to donate.

I'll have more to say on substance tomorrow, but it's getting pretty late. My tl;dr take would be that with pretty conservative estimates on total AI risk, combined with the lack of short term motives to address it (the threat of near-term and moderate scale bioterrorism drives research into defenses, not the fear of extinction-level engineered plagues; asteroid defense is more motivated by the threat of civilization or country-wreckers than the less common extinction-level events; nuclear risk reduction was really strong only in the face of the Soviets, and today the focus is still more on nuclear terrorism, proliferation, and small scale wars; climate change benefits from visibly already happening and a social movement built over decades in tandem with the existing environmentalist movement), there are still low-hanging fruit to be plucked. [That parenthetical aside somewhat disrupted the tl;dr billing, oh well...] When we get to the point where a sizable contingent of skilled folk in academia and elsewhere have gotten well into those low-hanging fruit, and key decision-makers in the relevant places are likely to have access to them in the event of surprisingly quick progress, that calculus will change.