Bizarrely, I think every single Rhys I've known (quite a few as I'm a teacher) has been an only child!
I love Zara.
I don't think it needs a Welsh name to go with it, unless you are Welsh and specifically want one. Rhys is much more widely used than the majority of specifically Welsh names and probably a lo of people don't even know it is a Welsh name. I see Rhys as going well with:
Anya, Autumn, Aria, April, Amaya, Bronte, Bella, Connie, Chloe, Clara, Daisy, Darcey, Erin, Elodie, Ella, Eva, Freya, Gracie, Holly, Hallie, India/Indie, Imogen, Jasmine, Kitty, Lola, Layla, Lyla, Leah, Matilda, Molly, Millie, Mia, Nova, Olivia, Phoebe, Piper, Poppy, Ruby, Rosie, Sadie, Summer, Scarlett, Sophie, Thea, Willow, Zoe
If you did want a Welsh name then I'd go for another one well known in ENgland like:
Alys, Bethan, Carys, Nia, Seren, Tegan