Maybe I have my bash/readline vi mode configured specially to do this, but if I want to delete the entire line and type a new one (from anywhere in that line), I do something simpler than either of these alternatives:

<esc>S

Esc exits insert mode (of course) and capital S erases the line and puts you in insert mode at column 0 (just like in (n)vim, right?).

Like I said, maybe I configured that? But 'S' is standard vim-stuff... (I'm not able to double check my config at the moment).

[Edit: right after hitting submit I realized that my way is perhaps "arguably" simpler because I do have to hit shift to get capital S. So I'm also hitting three keys...]

<c-o>S is also a vim sequence. The equivalent readline/emacs is <c-e><c-u> or <c-a><c-k>, or just <c-u> or <c-k> if you're already at the end/start of the line.