It was a joint effort.
My general view of the Final Frontier was that it is actually a decent movie but the troubled production and confusion throughout means they really needed one or two more script treatments with someone who actually knew what they were trying to say and be consistent about it.
Really, I think they just needed to make the following changes.
1. Establish Sybok as a proper villain (albeit a well-intentioned one) who is an immensely powerful psychic even by Vulcan standards, someone Spock wouldn't speak of (because he warps people's minds and is a psychic rapist even if he thinks he's helping them), and that he has reason to believe he's giving the galaxy a God to worship. Most of it is in the script already but it's scattered about.
2. Give Uhura, Checov, Scotty, and Sulu all their own little character bits because it's literally a movie where the bad guy gets into all their heads.
3. Remove some of the genuinely stupid.
4. Give at least a short explanation for "God."
"What was it, Spock?"
"An immensely powerful alien life form imprisoned long before the human race evolved from animals. My brother was capable of sensing it with his powers and was manipulated to its own ends."
"Might it have some connection to ideas of God across cultures?"
"No, Jim, because it wouldn't need a starship."