You can't say God is the source of morality if god's morality derives from secular morality.
It is common for secular people to say that their morality is a concept based on a concern for human well being. This idea is what I am calling secular morality.
It is common for Christians to say that their morality is based on God's laws, or God's rules, or God's mind, or God's nature, or something along these lines.
But if God's moral rules or moral nature is based on a concern for well being, then it would be flawed to say that God is the source of morality. God would be just applying the logic of secular morality, so he wouldn't be the source of morality.
God could still be the ultimate most trustworthy moral authority as he could use his omnipotence to always know what is best in terms of human well being. But this would just make him perfect in his application of secular morality, not the source of morality itself.
So if God's rules are just his application of secular morality we would expect God's morality to always line up with a concern for human well being. We would expect that his moral judgments would always align with what is best in terms of human well being. For example, God says do not murder because he knows that murder is detrimental to the victim, their family, their society as whole, and almost always to the murderer as well. So God says it's immoral because he knows it's detrimental to human well being. He's just applying secular logic.
Whereas is God is a separate source of morality we would expect to see some divergence from secular morality. There should be some instances where God's morality would dictate that something harmful to well being would be morally good (or conversely that something beneficial to well being would be morally bad). But are there actually any examples of this?
If there are no known examples of anything God considers both morally good and detrimental to human well being (or vise versa) then I think it is reasonable to conclude that God is just applying secular morality. Without any knows examples there is no way to distinguish secular morality from God's morality and that would make it reasonable to conclude they are the same.
So this leads to the question for Christians: Is there any example of something that you think God views as good and that you also think God views as being detrimental to human well being (or vise versa)?
I don't claim to be able to know all the possible examples or scenarios people can think up, but since the rules of the forum require the person posting to take a position on the topic, I will for the sake of argument assert that there are no such cases and that God's morality is therefore just an application of secular morality. And my way of supporting this position will be by responding to anyone claiming to have such an example, or anyone pointing out any other flaws with the reasoning of this post.