You don’t know anyone who heard Paul teach. They are long dead.
However, God would not fail to include all his necessary teaching in the Bible, or as you say, “It is impossible for Christ to have failed to keep all his truth available to the world.”
Better answered by Fathers Rumble & Carty that by me:
588. I admit the force of Apostolic traditions for the early Christians. But they could be sure of them as we cannot today.
That is a dreadful statement. Were the Apostolic traditions part of the Christian faith then? Is it therefore impossible to know the full Christian truth now? Did Christ mean it when He said that He would be with His Church all days till the very end of the world? Or would you suggest that He meant it, but could not accomplish it? He sent the Church to teach all things, yet you say that it is impossible today. Be sure that the Catholic Church has all necessary traditions embodied in her teachings. Within her fold each succeeding generation of Bishops have taught faithful men who have been fit to teach others also. But you refuse to be taught by that Church. You rely upon your own fallible judgment. And as long as you adopt that method you will never be sure, not only of the Christian traditions, but even of the true Christian doctrine to be derived from Scripture itself.