" The bottom line it's not everyone picking and choosing for themselves inventing doctrine rather than receiving it via apostolic authority."
But that is exactly what Augustine did in his arguments with the Manichaeans and Donatists. None of the doctrines used against them had been articulated until Augustine interpreted them from the scriptures, just as believers can today by the illumination of the Holy Spirit.
On the contrary Augustine wasn't picking and choosing. He accepts all of the Gospels. He's engaging in apologetic argument quoting Paul and sections of Scripture. Secondly as a bishop, Augustine is a successor to the Apostles and forms part of the teaching-authority of the Church. Thirdly Augustine was a theologian and sought to think with and in the Faith of the Church which he accepts in its entirety. Marcion would be an example of picking and choosing according to his own head. Hence he rejected the O.T. and rejected most of the N.T. except for Luke and Paul.
What else did St. Augustine have to say? About Church authority? About the Real Presence of Christ in the Eucharist? About the Virgin Mary? Non-Catholics can not just pick and choose from St. Augustine either. They must take him as a whole and he certainly was no Protestant who held to Sola Scriptura.