I know that support mythics are more versatile than the ones for other roles, but u really want to go Shurelya's 80% of the games on Janna, she's a disruption champion and the bonus ms she gainst and gives by simply shielding allies help the adc kite better, your team engage, make picks, or even fall back when caught out, I like the moonstone in cases when the fights look like they'll drag on, cause there are a lot of fighters/tanks, of even imperial mandate if your team needs some dmg boost, but I think rushing shurelya's is a great option cause this way you get abnormal boost in utility, especially since she doesn't need early boots cause of her w passive
Well guess you could go for mandate rush if you're stomping in lane and want to keep playing offensive, but still, 80% of time Shurelya's rush even before buying any boots is the way to go