How to prevent MacOS Mail autocomplete from inserting new line when accepting its suggestion?
Use SPACE to accept autocorrect suggestion.
If you like to use enter, then you can open dropdown list of suggested words with ⌥ + ESC, navigate to needed word using arrow keys and press Enter (will take cursor at the end of word).
Word you are looking for may or may not be first in list and automatically selected. If this happens to be true then of course no navigation with arrow keys is needed and enter can be pressed right away (⌥+ESC and Enter).
If you use SPACE instead of Enter then space is added between cursor and end of entered word so you can continue writing right away.