> Or to esc. Saves my vim pinky a lot of grief.

FWIW, Ctrl-[ emits the ASCII code for ESC. So you can save that 'vim pinky' and have the remainder of control codes available to you as well :-).

I forget what the tool was, but I recall someone mentioning they mapped CapsLock to ESC when tapped, and to Ctrl when held.

It's Karabiner Elements on the Mac.

On Linux you can get it to do the same thing with a combination of xmodmap, setxkbmap and xcape (but it's fiddly) and on Windows with AutoHotkey (but it's fiddly) — neither are as easy to use as Karabiner Elements, but they do work.