Ottilie: Catchy, stylish, stands out, good caboose name (If it is a caboose) because it doesn’t settle.
Charlotte- Well established and great at any age, but there are so many of them now, that she may have 3-4 in a class.
Caroline- Beautiful classic. Not too growny. Works at any age.
Rosie- It’s sweet. She sounds like the baby of the sibs. I prefer Rosemary.
Posie- Fun name. More intriguing than Rosie but less serious.
Etta- I’ve loved Etta for a long time. My favorite on your list.
Mn-
Ottilie Madge, Ottilie Ann, Ottilie Charlotte, Ottilie Clare, Ottilie Pearl, Ottilie Sarah, Ottilie Marla, Ottilie Cosette, Ottilie Cora, Ottilie Blanche
Charlotte Rosie, Charlotte Marian, Charlotte Daisy, Charlotte Nora, Charlotte Posie, Charlotte Jewel, Charlotte Ivy, Charlotte Ann, Charlotte Nadia, Charlotte Olivia, Charlotte Adelaide
Caroline Joy, Caroline Faith, Caroline Greta, Caroline Frances, Caroline Jo, Caroline Mercy, Caroline Camille, Caroline Thea, Caroline Bliss
Rosie Meredith, Rosie Caroline, Rosie Gretchen, Rosie Anabel, Rosie Maude, Rosie Marian, Rosie Ann, Rosie Amanda, Rosie Lauren, Rosie Jacqueline
Posie Kimber, Posie Olive, Posie Grant, Posie Ella, Posie Minette, Posie Rachel, Posie Annette, Posie Lorraine, Posie Blair, Posie Flynn, Posie Nell, Posie Ruth
Etta Charlotte, Etta Caroline, Etta Rosemary, Etta Jocelyn, Etta Renae, Etta Jean, Etta Millicent, Etta Beatrice, Etta Isabel, Etta Abigail
I’d certainly choose Etta, but if you’re still deciding, the following names fit your combined style-
Thea, Gemma, Rosemary, Holly, Bethany, Ivah, Natalie, Eliza, Jillian/Gillian, Romilly, Claudia, Constance, Fiona