Andrew, you say:

"But I would still maintain that in Paul’s
thought the transformation of the individual occurs within a bigger
story about the transformation of a community. This seems to me clear
enough from Romans 9-11: Paul thinks of the Gentiles as having been
attached to a saved remnant of Israel."

I’m neither equipped nor inclined to turn expand the scope of my engagement from the the "new creation" theme to a systematic theology of the New Testament, so I’ll offer only some tentative thoughts about Romans 9-11. Paul explicitly addressed this letter to a Gentile audience. In chapter 9 he shifts his attention to my brethren, my kinsmen according to the flesh, who are Israelites. There’s no way of knowing for sure, but I could imagine that the Gentile believers wondered whether, in light of the "new creation," God had abandoned Israel. As he did also in the Galatians letter, Paul shifts the temporal context back in time, from Israel to Abraham, emphasizing that

it is not the children of the flesh who are children of God, but the children of the promise are regarded as descendants (Romans 9:8)

In other words, the Abrahamic promise isn’t fulfilled through the biological "be fruitful and multiply" apparatus of the old creation — the means by which the nation of Israel attained distinction — but through some other channel altogether. Specifically, the biopolitical collective entity called "Israel" is not that channel, and it never was. Why? Because the channel passes through Christ and is apprehended not by biological inheritance nor by moral superiority but by faith in a resurrected Christ — the same channel by which Paul’s Gentile readers have entered into the new creation.

For there is no distinction between Jew and Greek; for the same Lord is Lord of all, abounding in riches for all who call on Him. (Romans 10:12)

Paul begins chapter 11 by distinguishing a chosen and faithful remnant of Israel - a microcosm within the microcosm you might say. But he says this narrowing of Israel is a temporary measure, intended to make possible the expansion of the promise far beyond the geographic and ethnic boundaries of Israel. When Paul speaks metaphorically of the olive tree (11:17ff.), he’s again referring not to Israel according to the flesh but to the descendants of Abraham according to the promise. While some of the "natural branches" — i.e., Israelites according the flesh — have been pruned from the branch, they can be grafted in again through faith.

…a partial hardening has happened to Israel until the fulness of the Gentiles have come in; and thus all Israel will be saved. (Rom. 11:25-26)

Is Paul saying that "all Israel" is a newly-pruned olive tree of the spirit, consisting of a faithful remnant selected from among Jews and Gentiles alike? Or is he saying that all Israel according to the flesh will eventually be reconciled and regrafted into the spiritual descendants of Abraham, along with the "fullness" of the Gentile descendants? It’s hard for me to say, but Paul wraps up his excursus on Israel, embedded within this longer letter to the Gentile believers in Rome, with this:

For just as you [i.e., Gentiles] once were disobedient to God but now have been shown mercy because of their disobedience, so these [i.e., Jews] also now have been disobedient in order that because of the mercy shown to you they also may now be shown mercy. For God has shut up all in disobedience that he might show mercy to all. (Rom. 11:30-32)

As I read this extended passage then, Paul contends that the pruning of the Israelite branch down to a remnant constituted a temporary measure. The pruning was implemented in order to make possible the explosive growth and flowering of the whole tree, Jew and Gentile alike, fulfilling the expansive promise made to Abraham long before Israel had even sprouted. Participating in this promised expansion — call it the "new creation" — is achieved by faith in God’s grace and mercy bestowed despite disobedience, or even because of it, through the death and resurrection of Christ.

More later hopefully…