I agree - there are times when it would be nice to have an option to “Turn this mirrored instance into a copy”. (ie - break the mirror / remove the link).
But as you note; yes, the way to “break” the mirror is to delete it.
When you delete a mirrored item; you’ll get a pop up asking if you want to delete:
THIS ITEM - which will delete only the instance you have selected and leave all others intact.
ALL ITEMS - which will delete ALL the instances of the item
CANCEL - close the dialog without doing anything