Emily - I like it but don't get excited about it. I don't think it's because it's too classic or too popular because I love lots of other names in those brackets. You can't go wrong with it though.
Tessa - I think I like this but go back and forth a bit. I don't like Tess much
Niamh - Really like it
Pippa - Really like it. Though might christen as Phillipa just for a full name option.
Eliza - it's ok.
Penelope (Nel) - not keen
Florence (Florrie) - quite like it. Not overly keen on Florrie (but better than Flo)
I know Williams with sisters called:
Fiona, Alexandra, Antigone, Fionnhuala, Eleanor, Matilda, Lydia, Ocean and Charlotte
Others in a similar vibe to your list that I think go with William:
Astrid, Anna, Annabel, Abigail, Bryony, Cassandra, Ciara, Elise, Emmeline, Edith, Frances, Grace, Gabriella, Georgina, Hannah, Harriet, Imogen, Juliet, Julia, Joanna, Josie, Leona, Miranda, Natasha, Natalie, Orla, Phoebe, Rebecca, Rose, Sophia, Tara, Thea, Victoria, Vivienne, Zara