I don't rule out the possibility of advanced civilization in the galaxy. The idea of a galactic civilization seems silly to me. Vastness of the galaxy you mentioned is one point. My point is why?

The Europeans looked for India and China because that is where the good stuff was and they wanted it. I think we imagine aliens would be explorers like Europeans.

The way more advanced Chinese had an earlier age of exploration in giant ships. They were generally unimpressed with what they found. Everywhere else was poorer and less advanced. They halted further exploration. An advanced alien civilization might be like Chinese and see little point in exploring the entire galaxy. The knee-jerk answers are more resources or curiosity.

• How many planetary systems would an advanced civilization actually need?

• How many planetary systems does an advanced civilization need to explore before it's been there, done that?