You need to play the game pretty much two times (well, not quite, but close to anyway) to get all the talk topics.
First, you should make one save before you get Cynthia's 12th shop event. And that save should be before you return to Grand Bell Hall (after the first IPD therapy event). This way you can easily get the Cynthia talk topic for each girl. For the first playthru, just give the Cynthia talk topic to either Luca or Cloche.
Then you should make another save before the path split. This one should be pretty obvious.
After that, just choose one of them and play, but don't select a girl as partner until just before the final boss. When you are near the end game, save the game and then dive into Jakuri to get all her talk topics that require high level Cosmosphere. Then defeat the final boss to get the Extra menu open up.
When that's done, reload the previous save and pick the Luca or Cloche (whoever you chose to protect) as partner and get all her talk topics that require high level Cosmosphere. You don't actually need to beat the game again unless you want to see the ending. As long as you save the game, the new talk topics you acquire will show up in the Extra menu.
After that, reload the first save and give the Cynthia talk topic to whoever you didn't give to. After getting the talk topic, save the game (so that it appears in the Extra menu).
Now reload the second save just before the path split. Choose to protect the other girl you didn't choose last time and collect the rest of the talk topics.