That's the next step. Once we agree that someone is right and someone is wrong, then we can start talking about the definition of "moral good". And that is a very difficult and complicated discussion. But just because it's hard to define doesn't necessarily mean it's not real.

You could for example take an utilitarian approach and then the objectively better decision would be the decision that leads to less suffering in total.

Simply because it is practically impossible for us humans to calculate the "total of suffering", doesn't mean this total does not exist. It objectively does exist for every given decision. Perhaps there are exceptions where there is equal suffering for all decisions. But that still wouldn't make it a subjective observation.

Arguably, the Aztec had an even bigger lack of information. For example by assuming that human sacrifices are a necessity. Or that women don't suffer when they are treated as lesser.

Suffering is an objectively "real" thing in our universe. Unless you also want to debate whether pain or the human existence is real.

This seems like an axiom of ethics: less suffering is good. Because why would more suffering be good?

This seems like it leaves us with the option to either decide actively against what is good, or make decisions randomly. Random would be if you don't consider whether a decision increases or decreases suffering / well-being. I am a total lay person for philosophy but this almost makes it seem like it's a logical fallacy to assume ethics (on a base level) are subjective. We must assume something to make a decision. And your decision always leads to an increase or decrease in suffering. Therefore all decisions are on an objective scale of mortality..?