The topic of the conversation should be expressed in one word within the conversation. So instead of making your model guess from a bunch of different combinations of chars, try instead to train your model to guess what word within the conversation is the topic. You probably shouldn't plug in the P1, P2, ... thing because the person speaking is not relevant.
This problem doesn't have to be solved using ML because there is already a traditional solution. Check this link: https://github.com/bnosac/textrank/blob/master/vignettes/textrank.Rmdhttps://github.com/vgrabovets/multi_rake