Now that I read the documentation thanks to reply by MailMate , I need to optimize my "Load Once” code.
One liner AppleScript in Keyboard Maestro:
tell application "MailMate" to perform {"loadImagesOnce:"}
References:
https://manual.mailmate-app.com/custom_key_bindings
https://manual.mailmate-app.com/key_binding_selectors.html