this is an incredible sound set. Thank you.

best sound set.

very nice, thanks!

I am glad you are thinking of that as well. For what it is worth, I think option three is the most seemless and as you said it keeps with the simplicity. A line at the bottom would be an interesting implementation though. Another idea would be to slightly indent or have a visual cue such as a bullet or some other unicode symbol on the first line of every message, if that is possible. (I like Unicode F847)

one suggestion, add some sort of bullet point that differentiates the line break upon sending the message and the one that occurs when you type something longer than one line, it can get a tad confusing.