“Righteous Gentiles are grafted onto Israel, and remain, if they continue in His goodness.”
Though close, that’s not quite correct.
Believing Gentiles have been made righteous and grafted into an olive tree in order to bear fruit. Christ is the root of that olive tree, not Israel. Israel is represented by branches, some of which were broken off because of unbelief. But Israel will be brought to repentance and faith as a nation, and the branches will be grafted back in again. See Romans 11.
It is true that Jesus ministered primarily to Israel. However, there were believing Gentiles and Samaritans who received Christ’s blessings and healing. The scriptures you cited contain a mention of this. In other places He marveled how foreigners had more faith than the Israelites.
Paul was wrongly accused of teaching that God had abandoned Israel and broken His promises. He makes a special point to correct this libel. Paul pointed out that he himself was an Israelite. But, as a nation, they remain in unbelief.
God dealt with them as His chosen earthly people. This was and is a special position of favor God has given to the Jews. However, it is not favoritism. To whom much is given, much is required.
We do see signs of national repentance in the Old Testament. It never really happened to the fullest extent. There were times Israel cooperated with God nationally. Israel once promised to keep God’s laws, but they didn’t (and couldn’t but needed to realize that). There were times when Israel cried to God to rescue her. But the day when all of Israel is going to be saved has not arrived yet.
Likewise, no other nations fully repented and turned to God either. We do see Nebuchadnezzar making a decree to honor the God of Israel throughout his empire. We do see the repentance of Nineveh, the capital of the kingdom of Assyria. But God has not dealt with any nation like He has with Israel, no nation has had the privileges of God’s revealed knowledge to the extent Israel has.
And Gentile believers have received many privileges and benefits through our connection with Israel in Christ.