You can also use xmodmap [1].

My xmodmap file has things like:

  ! Make Caps Lock be Escape and Scroll Lock be Caps Lock
  remove Lock = Caps_Lock
  keysym Caps_Lock = Escape
  remove Lock = Scroll_Lock
  keysym Scroll_Lock = Caps_Lock
  add Lock = Caps_Lock

After reading this post, I also reverted backspace and bar/backslash:

  keysym BackSpace = backslash bar
  keysym backslash = BackSpace

[1] https://wiki.archlinux.org/index.php/Xmodmap

EDIT: xmodmap link

XKB options have the advantage of also being understood by Wayland compositors.

Yeah. That’s cool, but I’m talking about conditional context switching.

ESC if pressed once.

CTRL it pressed with another key.

This is the sort of thing I'd want to modify the keyboard firmware for. QMK has explicit support for this kind of thing, for example: https://docs.qmk.fm/#/feature_advanced_keycodes?id=mod-tap