Is the new covenant only for Israel and Judah, or for everyone?

You are reading the text closely, and the text does say "the house of Israel and the house of Judah." That is exactly right, and you should not let anyone blur it. And you are right to resist a lazy "it is just for everyone" that quietly erases Israel. And the instinct that the gospel reaches the nations is also right; both are true at once.

So the error to refuse is the either-or, either Israel's covenant or humanity's. It is not one against the other. Jeremiah names Israel and Judah, and that stands. But Israel was never the boundary of the promise. She was the conduit of it. It runs back to Genesis 12:3, "in you all the families of the earth shall be blessed." The covenant cut with Israel is precisely how that promise to the whole earth finally gets kept. The nations come in not by replacing Israel but by being grafted into her own cultivated olive tree, "do not be arrogant toward the branches" (Romans 11:17-18). Worldwide, yes, but through Israel, never around her.

Do not take it from me. Read Genesis 12:3 and Romans 11:17-18 together, and ask whether the nations replace Israel or are grafted into her.