I dealt with the excess sweetness by swapping out the grenadine with pomegranate juice, which I bumped up to 1/2 oz, plus a couple dashes of orange flower water. So basically grenadine without the extra sugar.
I just made this with sugar-free Coke and grenadine. Totally understand the approach of dialing down the diabetic charge. I don't know about you, but I feel like this is an interesting twist on a Cherry Coke. Pretty good.