The default sounds good to me, but I wouldn’t make sending on shift+enter an option, because it’s like giving the option to use the ESC to submit things instead of preventing submitting, which will train muscle memory the wrong way in some users.
It’s like ESC to exit or ctrl+k to search — not an official standard, but it’s been done that way for so long by so many programs, that’s what users expect. Like you mentioned above: “This seems to be a pretty standard setting in all popular chats from Telegram to Element – i.e. to allow “Cmd+Enter to send, Enter for newline”.”
If people train muscle memory that shift+enter sends messages, they are going to get unpleasant surprises when they find out what happens in other programs.
I don’t use the two programs you mentioned, but Slack, Google Chat, Skype, Apple Messages, and most others use enter to send and shift+enter to create new lines. When enter doesn’t submit (Gmail, Discourse posts, etc.), ctrl/command+enter submits.
An option to turn off submitting for enter makes sense, but I think using shift+enter to submit is going to cause problems for people who choose that without realizing that it’s going to create another muscle memory problem.