Regarding the berries, if there was just a gene that said, “don’t eat as much if food is running low”, the individuals that had that gene would simply be outcompeted by those that don’t. (Even though this is worse for everyone than if everyone slowed down.) In order for such a gene to spread, you’d need some kind of enforcement mechanism where individuals who defect are punished. That is to say, collectively beneficial behavior can emerge, but it must be fully reproductive to behavior that is selfish at the level of the gene.

I think altruism is similar, in that it can also be reduced the behavior that’s selfish at the level of gene, as you say.

So, overall, I think “maximize number of offspring” is definitely not quite right. The answer is more, “maximize number of copies of genes that persist”. (But even that is a little complicated as you can have gene/gene competition as in transposons!)