Respectfully, I think you're putting the cart before the horse. "Let" him propose with any old thing and then take your time! Or buy the diamond and put it in a basic (and, yes, "basic") YG band so you have all the time you need to design the ring.
The fact that you both already know this is coming takes the mystery away! I presume you are part of this generation that has witnesses to the proposal, etc., and the hand shots are immediately on social media. So much needless pressure! We never would have risked that. I was confident my future-wife would say yes...but you never know for sure until you ask...