I've never heard of Rhiannyd before but it's gorgeous! One of the nicest names I've ever heard. Love Arya too.
Names I consider strong but feminine:
Anna, Anastasia, Aurelia, Arabella, Beatrice, Bethan, Bronwen, Claudia, Caroline, Charlotte, Constance, Cerys, Emma, Emmeline, Eloise, Erin, Esther, Florence, Grace, Giselle, Harriet, Helena, Isobel, Imogen, Juliet, Josephine, Katelyn, Katherine, Lauren, Lois, Miriam, Madeleine, Miranda, Nicole, Natasha, Philippa, Rhiannon, Robyn, Rachel, Sarah, Suzannah, Sorrell, Tamsin, Victoria, Violet, Yasmin, Ysanne.