Congratulations! What gorgeous babies.
Lovely list of names. I don't see a problem with 'filler' middle names at all but also think you've got enough of a list to draw from there too.
From the list, I like:
William Henry
Alexander James
Annabelle Grace
Charlotte Rose
Isabella Rose
Other ideas in the same vein:
George, Lucas, Andrew, Christopher, Joseph, Isaac, Zachary, Nicholas, Nathaniel, Joshua, Thomas
Abigail, Beatrice, Natasha, Madeleine, Miranda, Juliette, Katherine, Matilda, Natalie, Lucy, Hannah, Rebecca, Victoria, Felicity, Mary, Francesca, Imogen, Harriet
Or, if you want to consider the 'classic but more unusual' route, how about:
Alistair, Benedict, Christian, Elliot, Frederick, Jasper, John, Michael, Owen, Patrick, Quentin, Solomon, Timothy, Vincent, Xavier
Arabella, Alannah, Bronte, Claire, Cassandra, Daphne, Emmeline, Giselle, Henrietta, Josephine, Jocelyn, Kate, Lucinda, Larissa, Marianne, Nicole, Nadia, Rowena, Rhiannon, Rosemary, Rosamund, Susannah, Sylvia, Vanessa, Vivienne, Xanthe