If the only reason we have existence (I meaning everthing that exists), goodness or its essence would have to be a part of something that already exists. For something to have a essence, or be good, it would first have to be in existence.
If God is not in existence then your defination of God would be a non existent being.
The dictionary defines evistence as the state of fact or existing,continuance in being.all that exists,Something that exists;being.
It surprises me that you’re an existentialist. Most existentialists are atheists or new age pantheists. After all, the universe has always existed, and therefore God is not unique in that regard.
God exists because goodness compels His existence. There’s no other reason. His existence is necessary.
How could he have goodness before he existed. Do I read you right that goodness came before God? You say his existence is necessary, then he is in existence, so you are saying he had a place to exist. What goodness created Him?