As someone else who also uses ^M for line return here's my reason.

I have the Caps key mapped to control, so ^M is 2 small finger movements as opposed to reaching for the return key which means either moving my (weak in my case) right wrist or moving my whole right forearm. It's a small movement to the return key but I find eliminating it really helps to reduce strain related pain when typing.

I also use ^H for backspace and ^I for tab for similar reasons. This also keeps my fingers on the home row.