Your cons are not cons in my book. Users are very interested in the reactions, comments and zaps to their own messages + they use their own messages more than anything else as reference points to know where they are in the conversation.

Full width (à la Slack) makes it look a lot less like a conversation and a kot more like a feed. (which might be desired in some cases). But very few people love it (lack of breathing room, storytelling and rounded bubble frame).