Control+F4 - ‘move focus to the active or next window’ is essentially that. (With the caveat that if your keyboard focus winds up on the menu bar or otherwise not on a window at all, control+f4 shifts focus back to the window, rather than switching windows. The main way to make that happen is with the other control-f-key shortcuts that no-one uses, though)

If you’re going to use it I’d probably rebind it in the keyboard shortcuts settings.