Susannah is a really nice name! :)
There's also
Sukie, Zuzu, Sansie, or
Nana. You can also try nicknames from different languages like
Zuzka (Czech, Slovak - pronounced like this -
https://forvo.com/word/zuzka/),
Susanita (Spanish, not really shorter but it's really cute), or
Suzette (French).