It all boils down to this point as far as I can see and at the risk of repeating myself here goes.
Is a concept objective or subjective?
Good is a concept.
Circle is a concept.
We tend very qucikly to agree with each other that a circle is a circle.
With good it is obviously much harder.
However, is the circle as a concept we all understand only understandable because we all "agree" to it or is it understandable because our thinking is able to comprehend the absolute truth of it. Whether I think it or not, a circle is surely true?
My premise is thus that once our capacity to think has developed to its fullest capacity, whatever that means, we will find an objective content for the concept good that holds for every single person in every single situation regardless of personal preference. The concept of good is just as objective as the concept of circle.
If I am not willing to give concepts an objective truth then of course I can never agree with this. However we must take into doubt every single aspect of our lives in this case, for what is then objective?
The circle exists whether I agree with it or not, whether I like it or not.
This is the same for "good".
That people cannot yet agree with each other what good is has nothing to do with the concept itself, only our inability to think it.
Once we can all comprehend the utter conceptual content of "good" there will be no disagreement just like we would never disagree with the fact that a triangle has 3 angles.