Switch focus of window with direct visual shortcut keys

I am using BTT like the small version of a Ratpoison Window Manager. I can arrange my windows nicely using several actions on my desktop and the other virtual desktop. All of this with short and precise shortcuts.

But when it comes to switching from one window to the other, I am currently doomed to use a list of windows and select one window of them or cycling to a bunch of windows...

What I want is that:

  1. I press a button for "switch focus"
  2. BTT shows me small overlays inside the windows of my current desktop with a letter. By pressing this letter I will instantly switch focus to this windows.
  3. The letters of the windows should be constant as long as the window is alive. Rearranging the window should not change the letter.
  4. There should be also shortcuts to switch to other desktops (directly and cycling keys) so that I can see the shortcuts of windows on other desktops as well
  5. The shortcuts are global so that I can jump from desktop 1 / window 1 to desktop 3 / window 2 directly once I know the shortcut letter.
  6. That should also work for fullscreen windows.

Goal is to be able to swtich from one window to another window without using being forced to use the mouse or always to navigate through any kind of "list".