Best, Quickest way to get those Live Photos to Still Single Image

[rich in distress]

Photos app calls Key Photo the static image that is presented. To change it, hit edit and go the the bulls eye icon, select from the footage, and select Make a key Photo. You can make the video shorter here if you like.
You could click the Live Photo rectangle on top to turn it off, but it would still be available (same size, etc.).
Hit Done when you are.
To flatten it, hit the share icon (rectangle and up arrow) and Duplicate. It will give the option to Duplicate as Still Photo.
Automator or Shortcuts apps could make this automatic I’m sure.