I don't see why you can't have 3 'a' endings if you like the names and you have 2 already. I think Nyla is lovely and goes with the others. Similarly, how about Nia, Nyah or Nadia?
Non 'a' names - Kiera, Zara and ...
Amy, Astrid, Abigail, Bronte, Cerys, Cherry, Daphne, Darcey, Elise, Effie, Fallon, Grace, Giselle, Harmony, Hazel, Ivy, Juliet, Katy, Lucy, Leonie, Maisy, Nadine, Nancy, Poppy, Phoebe, Quinn, Ruby, Rosie, Romilly, Sophie, Saffron, Sorrel, Tansy, Tamsin, Verity, Violet