Josephine Joy! Josephine Joy! Josephine Joy! It's so happy and pretty, I love it. And it's so beautiful with your other girls. I'm personally a fan of Posy as a nn, but pretty much all its nns are fabulous. :) Anna Caroline is my second choice (although I'd love the see Caroline up front, too! I think Caroline Eva would be lovely with your other girls... if you're still looking for other options, and your hubby is open to the idea of Eva as a MN. :)). While I know how very much you love Susannah, and I think it's gorgeous with your other girls, but Sophie-Claire, Sarah Katherine, Serena, and Susannah are just too close, imo. Especially Serena and Susannah.
My favourite is Susannah Joy, though I agree that having the 4 youngest all with S names is a bit too match... but I stil really like Susannah. I'm so glad I don't have to make the call on that one. My 2nd fav is Anna Caroline. I like Josephine's nns, but not a big fan of the name in its long form. However, whichever name you choose, your daughter will be beautifully named!
Josephine is lovely. So is Joy. I don't like them together. Joy could be nickname for Josephine (between Josie, Josy, Posy, and Joey(Dawson's Creek for those to young to remember) it definitely fits right in.) It's sort of like naming your kid Margaret Meg or Catherine Kate or Caroline Carly. I voted for Anna Caroline. I almost voted for Susannah Joy. Either would work. I would love Josephine with a new middle name. Good Luck!