Thought of a little bit more:

In Best of Both Worlds 1 it's kind of unclear what the Borg plans for Earth are, actually. The Borg collective says "Your archaic cultures are authority driven. To facilitate our introduction into your societies, it has been decided that a human voice will speak for us in all communications" when they decide to capture Picard. Locutus says "Your life as it has been is over. From this time forward, you will service us." I don't know if the Borg goal was assimilating everyone at that point (or if the writers themselves had decided). From those bits it sounds like they want the resources of Earth and a slave caste of humans. When Guinan is briefing the crew she says the Borg destroyed her world and scattered her people, but there's no mention of assimilation.

By Best of Both Worlds 2 assimilation is the pretty clear goal. Locutus says that "you will become one with the Borg. You will all become one with the Borg." Maybe the process of assimilating Picard was so revolutionary that it took the collective a minute to figure out how great it could be and then it became their entire driving force.