How to determine which keyboard shortcut combo is free to assign?
As @Tetsujin pointed out, 


  The beep doesn't mean 'oops, I've got a key conflict' it means 'the key command you just used has nothing assigned to it in this scope'.


After a restart, the (above) shortcuts worked in every app I tried. 

So after tuning your keyboard shortcuts, try to close and reopen the app in which it does not get triggered (beeps) or ultimately, reboot.

Also - there's no notification if you have an existing short cut and install an app that collides with that, so you may have to just remediate when an app or shortcut collide - defining another shortcut in that app to free your preferred use.