I think you confuse cultural norms and rules with the abstract concept of ethics. When you question whether or not something is objectively morally better or worse than something else, that's independent of different current norms and values.

It's the same as maths. If you have two black boxes one with three and one with fifteen potatoes in them, the box with the fifteen potatoes in it will objectively have more potatoes. It doesn't matter if you can look into the boxes, like potatoes or if some people consider "less is more" or if they do not have a concept of mathematics. If you ask whether or not 15 potatoes are more than 3 this will always be an objective truth since numbers are defined by math and that a bigger number is bigger than a smaller number is an axiom of mathematics.

If the black boxes would instead contain "suffering" it doesn't matter what makes the suffering grow bigger or smaller. Someone who likes pain or for whom pain leads to greater outcome down the road will put less suffering into the box. It will still be an objective amount. Independent of whether or not you can measure it.