What was the hardest thing about switching to ed(1) for text editing?
Understanding that the atom of data in ed is not the character of text, as it is with WYSIWYG text editors & word processors. The atom of data in ed is the *line* of text. Every ed command does things with or to a line. Even "s".
This was also the case in the ed-inspired microcomputer editors MS-DOS Edlin & (mostly) CP/M Ed.
How do you enable auto-save while writing in ed(1)?
Muscle memory. Every time you reach for your drink, type period, enter, "w", enter, "a", enter.
... You *are* remembering to hydrate, right?