A very well. Ill give you my take on it.
Why does the existence/nonexistence of a God(s) matter?
I would argue that the existence/nonexistence of a God(s) is irrelevant to the pursuit of moral action.
The existence of God matters for much more than simply moral reasons.
If something is moral because God says it is moral, then does the claim "God is good" have any relevance?
Yes it does. let us take the 10 commandments for the prime example. 7 of them are good generally accepted moral standards. If people follow those 7 (the remaining 3 are Keep the sabbath day holy, no other Gods, and Using the lords name in vain) then people would consider them moral regardless of religious affiliation. The same applies to God.
Any being that dictates morality is unable to act in an immoral way. This further forces God to not be omnipotent if they dictate morality because they are unable to act in an immoral way. So if God dictates morality then God is neither omnipotent nor all Good.
Dictating something, and following it are 2 different things. You shouldnt make the assumption that just because something is dictated, the dictator follows it. The dictator can do whatever he wants for he has no one to answer to. In the same way, God does not have to follow the morals he sets in place for us. He chooses to because he is good, however, he could easily not follow them.
If God says something is moral because it is moral, then why keep a middleman? Why not seek and understand why things are moral or immoral so that you can better pursue moral action than by instruction alone?
What is the middleman that you are referring to here?