I would have pronounced it Ny-ah. But would only need correcting once. It's pretty. And it's like Mia/Myah/Maya, Kia/Kyah/Kaya etc. With the 'aia' spelling you'll have to tell people. I know a Nia (Nee-a) and a Nyah (Ny-ah) which might be more obvious spellings if you want?
Love Marla. One of my best friends is called that but spelled Mala so I guess that's another one open to interpretation!
Other ideas:
Ayla, Aria, Ariel, Alba, Ailsa, Aya, Ada, Bronte, Brynn, Cleo, Cara, Ciara, Dana, Darcy, Effie, Etta, Enya, Ellis, Elise, Erin, Eden, Farrah, Fallon, Faith, Fern, Faye, Fleur, Grace, Giselle, Gina, Hanna, Heidi, Hope, Hedda, Hebe, Hattie, Isla, Ilsa, Ines, Iona, Ivy, Iris, Ida, Ingrid, Juno, Jada, Jaya, Kayla, Kiki, Kyah, Nora, Nina, Orla, Piya, Piper, Quinn, Ruby, Rosa, Robyn, Romy, Rowan, Ruth, Rivkah, Rhiann, Sara, Sadie, Stella, Seren, Tia, Thea, Tara, Tessa, Tamsin, Una, Veda, Willa, Zia