If we were redeemed from the curse of the Law (and the "us" really means us) as it would in the context, then it seems that Jesus becoming a curse for us has a direct meaning - beyond covenant theology.
I do not believe the "us" being redeemed from the Law by Christ be coming a curse under the Law (the Mosaic Law statute which says men hanging on a tree were a curse) can be applied to us in that sense.
The reason is Gentiles were not under the Mosaic Law (and after the cruvifixion no man was under the Law of Moses).
My opinion is that like the Law shows us our sin this is equated to Christ being made sin for us.
But yes, the "us" refers to those under the Law of Moses (the same Law that states those hung on a tree are a curse). This canceled the Law. This liberated those who were curses under that Law. That certificate of debt was canceled, hung on a tree.
You are forgetting that salvation came first to the Jews and then the Gentiles (this being a historical order). But the Old Covenant had to be canceled for the New to come.
We were not redeemed from the Law of Moses. Christ died to redeem us from "the bondage of din snd death", from "this evil world".
You are forgetting that we (assuming you are not a pre-Crucifixion Jew) were never made a curse under the Law - BUT be ause of din if we were placed under the Law we would be a curse.
Paul's argument in that verse, however, is not about atonement. You are extracting one verse and ignoring Paul's argument entirely.
Paul's purpose was admonishing the audience to sn extent through correction regarding falling under the Law of Moses.
He introduces the problem by relating his calling out of Peter for trying to place Gentiles under the Law (2:15) and states it is a transgression against the law of Christ to rebuild that which has been destroyed.
1. In the actual argument Paul starts with the Jews who were made a curse under the Law.
2. He then moves to Jesus redeeming them from that curse so that the blessing of Abraham might come to the Gentiles.
3. The argument concludes that this was do that all would receive the promise of the Spirit through faith. (Christ was creating in Himself one new humanity).
4. Paul then explains the intent of the Law in relation to the superior Promise (the covenant made with Abraham).
5. The Law ame 430 years after that covenant of the Promise and was to Abraham (apart from the Law) and his Seed singular, Christ).
6. The Law was added because of transgressions, having been ordained through angels by the agency of a mediator, until the seed would come to whom the promise had been made.
7. Scripture has shut up all things under sin, so that the promise by faith in Jesus Christ might be given to those who believe.
8. The Law did not impart righteousness but was a tutor of Christ.
9. The Law is canceled. The Jew and Gentile division is no more. There is one people of God.
That is Paul's argument up to Chapter 5.
You are not the only one guilty of pulling out one verse out of Paul's argument to try and make a different point. I did at one time, and many still do because the words sound good and can be applied many ways. BUT that verse is given in a different context.
Christ becoming a curse to cancel and fulfill the Law is not our salvation itself (even according to PSA). You do not even believe that the canceling of the Law equates to salvation (I hope).
We have to be more respectful to God's words than larger mainstream sects are willing.
You would make the same observation (I think) if the verse was "do not judge" and the claim was Christians cannot identify sin because that is not what those words mean in context of the argument.
But you are doing the same thing with one verse here (you are taking it out of Paul's argument to give it a new meaning).