> I think I have about a 50% success rate in typing a multiline message without accidentally sending it early, having to edit / copy and delete the message and finish typing it before the recipient has a chance to read it, confusing both me and them in the process.

That won't change if you had a different dedicated key for "move one line down but don't send `enter` keycode". You'd still accidentally hit `enter` due to muscle memory.

After all, if you could get past muscle memory, you'd simply press Shift while hitting Enter.