“The grafting described by the Apostle to the Gentiles is that righteous Gentiles are grafted onto the olive tree, among the natural branches, not onto the root itself. With the natural branches, they partake of the root and fatness of the olive tree.”
What Paul is describing is atypical. While this may have been done sometimes in the real world as a way to get natural olive branches to produce more fruit, the usual grafting was of natural branches from a cultivated olive tree into a wild olive tree. Why? Because olive trees take time to grow, and years to bear fruit.
But if you take a wild olive tree and transplant new olive tree shoots, the “fatness” of the wild, more mature olive tree will supply it with the nutrition it needs to bear fruit sooner.
Paul was writing this to Roman believers who were a unique group. This was the one city where a local church was established without any apostles directly doing so. Jewish converts to Christ who believed in Acts 2 went back to their homes in Rome and began meeting as a church. They won Gentile converts. Later, the emperor expelled all Jews from Rome, including the Christian Jews.
So the Roman church was, probably at the time of Paul’s writing, made up of Gentile Christians that had not been subject directly to the teaching of apostles. Some had become proud and began to believe they were superior to the Jewish believers. Paul uses language to demonstrate the interdependence of believers. Gentiles have a great privilege to become partakers of the “commonwealth” of Israel. God is using the salvation of Gentiles to provoke Jews to jealousy, to return to faithfulness to God by way of repentance and belief on Christ.
All analogies break down. I am not meaning to contradict your point other than to emphasize that Gentile believers are grafted into Christ. Our connection is directly with Him. We do not become Jews. It might be proper to say we are God’s Heavenly chosen people, the Israel of Heaven or Israel of God. But this includes all followers of Christ, Jews and Gentiles. The bottom line here is that we do not have access to Christ by becoming Jewish. We have access to the privileges enjoyed by Israel through our relationship with Christ.