I love Raya - it's a nice sound, it has the connotation of a ray of light, and it makes me think of socialist and feminist activist and theorist Raya Dunayevskaya. She was born in what's now Ukraine, in 1910, and changed her name to Rae when she moved to the US. Raya has a very Slavic feel to me - could be tied to Rachel, but doesn't have to be. Rae/Ray is a nice nickname. Nadia also feels Slavic.
I'd consider: Anastasia, Alicia, Alyssa, Anja/Anya, Aria/Arya, Celia, Freya, Julia, Lilja/Lilya, Maia/Maja/Maya, Natasha, Niamh/Neve, Sofia/Sofya, Sonia/Sonja/Sonya, Sylvia/Sylvie, Tania/Tanya, Thalia, Zoia/Zoe.
I have no idea about Harla (never heard it before - maybe a girl version of Harlan?) but if you like the sound - consider Alexandra, Alina, Bella, Calla, Carla/Karla, Carly, Charlene, Charlotte, Daria, Darla, Darleen, Ella, Fionnula, Kayla, Kendra, Leda, Lena, Lydia, Marla, Marlene, Marley, Marya, Nola, Nuala, Orla, Petra, Scarlet, Sandra, Starla.