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).
I really like Sansie! Thank you!I like Zuzu too but too close to Zazu and I'm a big fan of The Lion King haha. With Susanita or Suzette, I'd not really see the point of naming her Susannah even though they are both nice names. Thanks!