It is a common thesis here that most humans would ultimately have the same moral judgments if they were in full agreement about all factual questions and were better at reasoning.

That's not plausible. That would be some sort of objective morality, and there is no such thing. Humans have brains, and brains are complicated. You can't have them imply exactly the same preference.

Now, the non-crazy version of what you suggest is that preferences of most people are roughly similar, that they won't differ substantially in major aspects. But when you focus on detail, everyone is bound to want their own thing.