(people donating to to provide trips to Disneyland instead of buying personal luxuries)

This has incredibly marginal utility. It is effectively trading your luxury for the fuzzy feeling of providing luxury to another.

(people donating to x-risks charities instead of donating to provide trips to Disneyland)

This has more utility. In fact, it bears a strong resemblance to

(people donating to x-risks charities instead of buying personal luxuries)

given that "providing trips to Disneyland" looks more like a luxury than charity.

I don't understand how you can prefer A>C but C>A*, unless you think that "preventing the purchase of personal luxuries" is worth more utility than preventing existential risk (A, A*) or saving lives (B, B*).