And yet, through all of this, you haven’t quite explained how reasoning determines morality.
Let me say it this way,
Yesterday’s morality developed by reasoning isn’t today’s morality.
Right. Yesterday's morality developed by reason concluded that since the Bibkle says we can't tolerate witches, we must execute them.
Of curse today's morality understands that witches are not a threat, and that human rights prevail.
Or...
Yesterday's morality devoloped by reason concluded that black people were subhuman and could be enslaved, because the Bible says so.
Until objective reasoning arose in the Enlightenment and argued for human rights for all humans, and today's morlaity defends the rights of all humans.
Or....
Yesterday's morality developed by reason concluded that Jews were a threat to European Christians and could be rounded up and exterminated.
Of course today's morality understands that Jews are human and have rights, even if Christians see them as a threat of some sort.
Today’s morality, more than likely, will not be tomorrow’s morality after discussion and reasoning.
So what is wrong with today's morality, and why do you want a change?
When do we know when we have actually found true morality?
When religious and political dogma is not ruling a person's thinking. Have you noticed that atheists are among the most accepting of any group? Atheists oppose false beliefs, and they don't suggest theists be killed. Atheists ask believers to acknowledge their beliefs are personal and subjective, and have no authority beyond themselves. That's it.