Build keyboard shortcut for "minimize all windows of current app except the current window"

Solution 1:

The following example AppleScript code will minimize all but the front window in Google Chrome:

tell application "System Events" to ¬
    set miniaturized of windows of application ¬
        "Google Chrome" whose index is not equal to 1 to true

Since you already have BetterTouchTool and assuming it can run AppleScript code or an AppleScript .scrpt file with a keyboard shortcut, that is probably the easiest way to go to trigger the code.

That said, my preferred method for running AppleScript scripts with a keyboard shortcut, is to use a third-party application named FastScripts, however you can also use Automator with a Run AppleScript action as a Service/Quick Action and assign it a keyboard shortcut in: System Preferences > Keyboard > Shortcuts > Services

• The keyboard shortcut assigned to the Service/Quick Action needs to not conflict with a default keyboard shortcut for whichever application is frontmost at the time it's pressed, otherwise there may be unwanted behavior.

• _{FastScripts can be run as a free app, up to 10 keyboard shortcuts, or upgraded for $24.95 USD to unlock unlimited keyboard shortcuts. I have no affiliation with Red Sweater Software, LLC, other then as a user of FastScripts.}