Ah actually you might be right. I just checked my cassette and it turns out the smallest two cogs are not connected, which makes sense with a loose lockring. Those outer 3 cogs would produce the most torque (being furthest from the base of the cassette), so if the cassette was a bit loose, they would be the prime suspects for the squeak. As it turns out, my cassette wasn't quite at the 40 nm it was supposed to be (maybe more like 15), so hopefully this fixes it! I'll find out tomorrow.