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.
What about the ed(1) command prompt? It doesn't have one!
There are only two modes: command & entry. If you're ever less than totally sure about which mode you're in. just type a period (".") on a line by itself.
If you were in entry mode, nothing will happen, and you're now in command mode.
If you were in command mode, the current line is printed, and you're still in command mode.
Either way, you're now in command mode & your next command will act on the line you think it will.
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?