Paul doesn’t say that all national Israel shall be saved. Indeed he quotes Isaiah as follows:
All throughout the Old Testament days, there was a remnant of Israel who were true Israelites, and who were obedient to Yahweh.
In the days of Jesus, the remnant consisted of His disciples (not the 12 only, but all of them). The remnant has always been “the Israel of God.”
In Romans 11, Paul uses the figure of an Olive Tree to represent the true Israel. Those who were not disciples of the Messiah whom God had sent to Israel, were cut off from the Olive Tree and branches from wild olive trees (the Gentiles who became Jesus’ disciples) were grafted into the Olive Tree, the Israel of God. The Israel of God has never ceased to be, but has continued with some branches having been cut off, and others grafted in.
And in this way all Israel will be saved, as it is written, “The Deliverer will come from Zion, He will banish ungodliness from Jacob”. (vs 26, ESV)
Not “as so all Israel shall be saved” but “In this way, all Israel shall be saved,” the Israel of God will have those who don’t truly belong to it cut off, and those who have never belonged to it (the Gentiles) but who have become disciples of the Messiah, brought in. Thus everyone, all of the true Israel, shall be saved.
Then Isaiah is so bold as to say, "I have been found by those who did not seek me; I have shown myself to those who did not ask for me.”[That’s some of the Gentiles]. But of Israel he says, “All day long I have held out my hands to a disobedient and contrary people.” I ask, then, has God rejected his people? By no means! For I myself am an Israelite, a descendant of Abraham, a member of the tribe of Benjamin.God has not rejected his people whom he foreknew. Do you not know what the Scripture says of Elijah, how he appeals to God against Israel? “Lord, they have killed your prophets, they have demolished your altars, and I alone am left, and they seek my life.” But what is God’s reply to him? “I have kept for myself seven thousand men who have not bowed the knee to Baal.” So too at the present time there is a remnant, chosen by grace. (Rom 10:20,21, 11:1-5 ESV)
So God has never rejected Israel. For there has always been a remnant who were faithful to Him and who were the true Israel. The 7000 who did not bow to Baal were the remnant in the days of Isaiah. They were the true Israel at the time. But in Paul’s day “at the present time” the remant were the disciples of Christ, chosen by grace. That included both ethnic Jews and Gentiles who followed the Messiah of God.
I suggest reading Philip Mauro’s The Hope of Israel. Though Mauro seems to have been a preterist, whereas I am a historic premillenialist, I believe Mauro had the correct understanding of Israel, and Romans 9-11.
preteristarchive.com/Books/1922_mauro_hope-israel.htm