And if so, would that entail that the beneficial effects to the group can overwrite the wellbeing of a single individual?

Nope. This is contradictory with your earlier definition. If morality can't exist without humanity, then that means morality is subjective. If it were objective, it would apply across the board.

But if morality is also "Doing what is best for the group, even at the expense of the individual" then you've just defined it as objective, not subjective.

The reality is, morality is subjective. But it's not chosen. It's part instinct that derives from us being social animals, but it's also part custom, tradition and culture. There is no objective morality, sure, but that doesn't just mean that folk get to remove other peoples agency and claim the moral high ground.

How can you find certainty in what’s morally acceptable and what is not?

You can't. You just have to listen to your own sense of right and wrong. If it's too much at odds with the rest of society, you'll know about it...