I am of course aware of Jeremiah 31:31, but Jeremiah was also a prophet sent to Israel.
The new covenant will be made with Israel the nation as jer 31:31 and heb 8:8 stated, so you cannot simultaneously reject replacement theology and believe that it is already in place at the same time.
In fact, Paul wrote this in Galatians 3-
6 So also Abraham “believed God, and it was credited to him as righteousness.”
7 Understand, then, that those who have faith are children of Abraham.
8 Scripture foresaw that God would justify the Gentiles by faith, and announced the gospel in advance to Abraham: “All nations will be blessed through you.”
9 So those who rely on faith are blessed along with Abraham, the man of faith.
10 For all who rely on the works of the law are under a curse, as it is written: “Cursed is everyone who does not continue to do everything written in the Book of the Law.”
11 Clearly no one who relies on the law is justified before God, because “the righteous will live by faith.”
12 The law is not based on faith; on the contrary, it says, “The person who does these things will live by them.”
13 Christ redeemed us from the curse of the law by becoming a curse for us, for it is written: “Cursed is everyone who is hung on a pole.”
14 He redeemed us in order that the blessing given to Abraham might come to the Gentiles through Christ Jesus, so that by faith we might receive the promise of the Spirit.
Now keep all this in mind as we continue:
16 The promises were spoken to Abraham and to his seed. Scripture does not say “and to seeds,” meaning many people, but “and to your seed,” meaning one person, who is Christ.
17 What I mean is this: The law, introduced 430 years later, does not set aside the covenant previously established by God and thus do away with the promise.
18 For if the inheritance depends on the law, then it no longer depends on the promise; but God in his grace gave it to Abraham through a promise.
19 Why, then, was the law given at all? It was added because of transgressions until the Seed to whom the promise referred had come. The law was given through angels and entrusted to a mediator.
And...
21 Is the law, therefore, opposed to the promises of God? Absolutely not! For if a law had been given that could impart life, then righteousness would certainly have come by the law.
22 But Scripture has locked up everything under the control of sin, so that what was promised, being given through faith in Jesus Christ, might be given to those who believe.
So, if you have understood these verses, your questions should be all cleared up. If not, I'm available for explanation.
If you do, then you have to answer the question, who is the Israel that the new covenant was made with now?
Again, I'm open to explanation, but I expect the verses are clear enough on their own.