so either civilizations are expending to less than 1000 stars on average, or they're not using radio waves, or our guesses about how common they are are wrong

Absent FTL communication, it is hard to imagine a scenario in which any central control remains after civilization has spread to more than a few stars. There would be no stopping the expansion after that, so the first explanation is unlikely.

A civilization whose area of expansion includes our own solar system would be perceivable by many means other than radio, so the second explanation is really not relevant.

That leaves the third as the most likely explanation, I am afraid.