First, Walking Ballista needs to have at least two +1/+1 counters on it (by paying four generic to cast it, ideally), and Heliod, the Sun-Crowned needs to be on the battlefield
Mystical Tether is a very solid white removal spell, and the instant speed mode makes playing against white quite more complicated
Kitsa has vigilance, prowess, and two abilities - you can tap the otter to draw a card and then discard a card, or you can spend two generic mana and tap it to copy an instant or sorcery spell you control, with the caveat that Kitsa needs three power to do so
1 Flicker of Fate
With regular copies selling for $80 minimum, and other variants breaking into three figures, Force of Will is the best thing you can open in a Secrets of Strixhaven play booster