We’ve chosen this (Peaceful, Quartz) countertop. Can we use this as the backsplash as well?

Peaceful is a great choice and beautiful color! Because the slabs are so thick, most people do not wish to use the material on the wall, but it is your choice! Just keep in mind that you will lose the space on the countertop with the thicker material sitting on it. It sometimes interferes with faucet placement as well. Hope this helps with your decision! Thanks for the inquiry!