Israel is a nation, not nations. You can join, Isaiah 56:1-8.I agree the new Covenant is strictly with Israel which some believing in a Replacement Theology (church replacing Israel) make me a cult.
However, I have to strongly disagree with your rejecting Yeshua being The True Messiah of both the two houses of Israel Who brought about the New Covenant.
I don't believe in a Trinity which has nothing to with either OT or a NT. Neither I believe in what overall Christendom teaches. Christendom also misunderstands 'Gentiles' to mean other than dispersed Israelites. The Hebrew word for Nations is the same Greek word as 'Ethnos' which some translators have translated as 'Gentiles' which have dual meanings. The only difference is that 'Gentiles' have no singular form 'Gentile'.
Israel was Nation. However, Abraham is the father of many nations. Why? It's because of the dispersion of Israel into many countries. It has nothing to do with a Replacement Theology that's popularly taught. God has limited His salvation plan to only Israel which transitioned from the Old Covenant to the New Covenant through Yeshua HaMashyach.
Since you don't believe Yeshua is God, what benefit is there in believing in him given that God saved him as well?
The new covenant is the same law written on the hearts and minds, Jeremiah 31:31-34.