But nonetheless, you say that 'petra' refers to Christ Himself, and He is a male, like Peter. Why does the word become 'petros' for Peter, but remains 'petra' for Christ?
For the same reason The One New Man is described in the feminine at the end of Ephesians.
While Peter, the at times prone to glaring instabilities would not have been, and was not The Rock.
Galatians 2:6 But of these who seemed to be somewhat, (whatsoever they were, it maketh no matter to me: God accepteth no man's person: ) for they who seemed to be somewhat in conference added nothing to me:
Galatians 2:11 But when Peter was come to Antioch, I withstood him to the face, because he was to be blamed. 2:12 For before that certain came from James, he did eat with the Gentiles: but when they were come, he withdrew and separated himself, fearing them which were of the circumcision. 2:13 And the other Jews dissembled likewise with him; insomuch that Barnabas also was carried away with their dissimulation. 2:14 But when I saw that they walked not uprightly according to the truth of the gospel, I said unto Peter before them all, If thou, being a Jew, livest after the manner of Gentiles, and not as do the Jews, why compellest thou the Gentiles to live as do the Jews?
By Acts 15, James was the one in charge at Jerusalem...
Acts 15:13 And after they had held their peace, James answered, saying, Men and brethren, hearken unto me: 15:14 Simeon hath declared how God at the first did visit the Gentiles, to take out of them a people for his name.
15:18 Known unto God are all his works from the beginning of the world. 15:19 Wherefore my sentence is, that we trouble not them, which from among the Gentiles are turned to God: 15:20 But that we write unto them, that they abstain from pollutions of idols, and from fornication, and from things strangled, and from blood.
While, missing from the list of key figures at Rome, near the end of Romans (written as late as it was) is Peter's name.
Acts 17: 11, 12.