> I'm still not very convinced any civilization will actually expand significantly beyond one solar system.

Whether a civilization expand beyond their solar system is entirely depending on if they develop the technology to do so in a relatively short time compare to their life span.

One good reason could be habitable planets. There may not be enough habitable planets left in the solar system.

Extra: some sci-fi look at Kardashev scale https://en.wikipedia.org/wiki/Kardashev_scale