Beginning-of-line or indentation on one key

mbol-advice.el

(defun my/mbol-advice (arg)
  "With prefix ARG other than 1, return t.
Else, go `back-to-indentation', and if point hasn't moved, return t."
  (if (eq arg 1)
      (let ((start (point)))
        (back-to-indentation)
        (eq start (point)))
    t))

(advice-add 'move-beginning-of-line :before-while 'my/mbol-advice)

This is some simple code to implement my preferred behavior on the "C-a" (or "<home>") key. It goes to the first non-whitespace character on the line (just like "M-m" would do). If pressed a second time (or if point was already at that location), it moves to beginning-of-line.

There are many other ways you could do this, even packages you could install for a more fine-grained approach (e.g. MWIM).

There's also a built-in feature that applies only in org-mode buffers. You can configure that by setting the value of org-special-ctrl-a/e. (The code in this gist won't interfere with it.)

Updated so that if a prefix argument other than 1 is passed, move-beginning-of-line will be immediately called using the prefix argument, since back-to-indentation doesn't use a prefix argument.