If you still plan to reroll, going for Dark Amaterasu would be better if you want to use a dark team, in fact, she's the highest priority for them.
As for your current set up, your goal should be getting D'art for Sniper Shot, because you don't have Dark Amaterasu and you already missed the Koihime event, thus you have no b-frame debuffs at all and because dark teams don't have any aside from those two, if you miss her, you're stuck using Sniper Shot for another year until the event comes back. Siegfied is bottom of the list, she's not even worth getting til the end, her mastery bonus is garbage and her ex skills aren't important.