Why not convert to Judaism and (at least) double that figure?
Because Catholicism is authentic Judaism.
Probably because Christianity is part of the equation here.
He obviously wasn't knowledgeable about his faith before he became a Roman Catholic. If he was, he would have known that the true church is the "ecclesia" the called-out assembly of believers in Jesus Christ. It's not any denomination or religion and certainly not one place or building. He could have stayed where he was and become a dedicated, justified and sanctified child of God by being saved by grace through faith in the sacrificial atoning blood of Jesus Christ.
He also divorced his first wife and married his mistress. Did he get an annulment?
It is true that the "faith" really goes back to when Adam and Eve were cast out of the garden. God made them clothing of animal skins. So even back then, blood was shed as a covering for sin. When the Messiah came, he was the "lamb of God that takes away the sins of the world". His blood was shed to make a complete and final propitiation (satisfactory payment) for our sins. Praise be to the one, true God Almighty!