Were the doctrines and revelations Paul received kept secret in God's mind before he revealed them?

Yes. The doctrines and revelations given through the apostle Paul were, for the most part, kept secret in the mind of God until He revealed them to Paul. Paul calls this a mystery, meaning it had never been revealed before.

The teaching explains that Paul says he is going to reveal a secret, or mystery, which means it had never been revealed before. It emphasizes that all the doctrines and revelations that have come to us through this apostle were for the most part kept secret in the mind of God until He revealed them to Paul. Earlier, in Luke 18, the Twelve understood none of the things being said because the saying was hid from them. That illustrates God's prerogative to keep things secret and hidden until it is time to reveal them.

1 Corinthians 15

Why does Paul reveal things the rest of Scripture doesn't touch on?

Paul reveals things the rest of Scripture doesn't touch on because it is in his writings alone that the true nature of the old Adam—the inherited Adamic sin nature—is made known. The Old Testament and Jesus taught the heart is wicked, but Paul alone explains this inherited sin nature from Adam, and he clarifies the resurrection order in 1 Corinthians 15.