Sydney is top 300 for girls (I think in the UK, maybe I've been playing too many US quiz games but I'm fairly sure it's UK) I don't know how popular it is for boys atm but it's a standard girl's name too now. I like it and it would go well with Marley.
I would maybe avoid another ee sound. I think Marley would go nicely with (sorry for any repeats):
Anna, Amber, Alexis, Alexa, Alyssa, Autumn, April, Aurora, Ayla, Aria, Alannah, Astrid, Aubrey, Anika, Brooke, Belle, Bethan, Brenna, Caitlyn, Claire, Ciara, Carys, Crystal, Danielle, Dakota, Dana, Emma, Erin, Erica, Eden, Elsa, Eva, Elina, Enya, Ellis, Farrah, Flora, Faith, Freya, Grace, Gianna, Genesis, Giselle, Hannah, Hazel, Harper, Iris, India, Iona, Jordan, Jayla, Jada, Jasmine, Kayla, Kyla, Kyra, Kendra, Kristen, Krista, Lauren, Lana, Lila, Layla, Lyla, Lyra, Luna, Leah, Lola, Lois, Louisa, Nia, Nyah, Nicole, Nadia, Nina, Niamh, Nevaeh, Orla, Olivia, Paige, Peyton, Pippa, Quinn, Rosa, Robyn, Samantha, Sienna, Savannah, Sasha, Taylor, Tia, Tianna, Talia, Thea, Tessa, Tara, Willow, Yvette, Zara, Zia.