What's the role of altruism in Buddhism?
Altruism is defined as the belief in or practice of disinterested and selfless concern for the well-being of others.
With respect to Buddhism, altruism is therefore:

AN4.95:1.4: one who practices to benefit others, but not themselves;

Quite significantly, Buddhism goes further:

AN4.95:3.2: The person who practices to benefit themselves, but not others, is better than both of those.

Importantly, those who practice to benefit themselves but not others are not practicing stinginess in the conventional sense. Rather, those who practice to benefit themselves but not others are in seclusion extinguishing defilements of self-involvement.
Ultimately, Buddhists ideally practice to benefit themselves and others:

AN4.95:3.3: But the person who practices to benefit both themselves and others is the foremost, best, chief, highest, and finest of the four.

When I first read this, I found it a startling departure from my preconception that altruism was the highest ideal.
A parallel consideration here is that altruism concerns itself with &quot;good deeds&quot;. Here, too, the Buddha says quite simply:

AN4.233:1.3: There are dark deeds with dark results; bright deeds with bright results;

But the Buddha goes on to say more:

AN4.233:1.5: dark and bright deeds with dark and bright results; and neither dark nor bright deeds with neither dark nor bright results, which lead to the ending of deeds.

We're all familiar with the muddle of dark and bright. But what is the fourth goal?

AN4.233:5.1: And what are neither dark nor bright deeds with neither dark nor bright results, which lead to the ending of deeds?  It’s the intention to give up dark deeds with dark results, bright deeds with bright results, and both dark and bright deeds with both dark and bright results.

That fourth goal leads to the end of suffering for all.