According to Jeremiah 25:11, the 70 years were the time the "nations will serve the king of Babylon". A time of servitude as vassal states, not of captivity in a foreign land.
Jeremiah 25:12 shows when the 70 years end. "when the seventy years are fulfilled, I will punish the king of Babylon and his nation". Babylon was punished in 539 BC.
Jeremiah 27:7, 8 reiterates this, "All nations will serve him and his son and his grandson until the time for his land comes; then many nations and great kings will subjugate him. If, however, any nation or kingdom will not serve Nebuchadnezzar king of Babylon or bow its neck under his yoke, I will punish that nation."
Jeremiah 27:11 shows that 70 years did not have to be one of captivity: "But if any nation will bow its neck under the yoke of the king of Babylon and serve him, I will let that nation remain in its own land to till it and to live there"
Jeremiah 29 was written to the Exiles who had already been taken captive under Jehoiachin. He tells them to settle down. That is why verse 10 says, “When seventy years are completed for Babylon, I will come to you and fulfill my good promise to bring you back to this place."
There would be no hope of return for the first or future waves of Exiles until the 70 Years elapsed first. Jeremiah is not talking about a 70 Years that would start at a future point, but a 70 Years that had already begun.
At this point, Jerusalem was not destroyed, nor did it have to be, provided it remain in servitude to Babylon during the 70 years.
Rather than reading a few verses, the real context comes from reading chapters 25 to 29. Only then does the 70 years have real context.