Hmm, let me air some thoughts, and see if it helps.
In general, moving from SR6 to Sixth World is a significant transition form an options-heavy game to an options-light game. The system is way less crunchy. So every PC archetype has 'fewer' options to exercise: a street sam doesn't have to decide anymore whether they want to move, activate, attack then move again, a rigger isn't counting his number of drone actions or speed of his car, a mage, as you rightly point out, isn't selecting from a range of 12 or more spells to find the perfect one to fit the occasion. Most combat will be over in two to three rolls. This can feel like a nerf. The positive flipside of the PbtA system is to think of what you want to do, rather than what the rules spell out you can do. Once you decide what you want to do, there should be a move that allows you to do it.
We should talk about whether Missy got translated well into Sixth World though, and also for Clint, Eunbyeol and Hotaru.
My understanding of your original concept for Missy is a black magic ritualist, thinking that she was betrayed by MCT. Now she has the opportunity to get to the bottom of this because the Shanto situation has dragged her back in to investigate his disappearance and solve her own problem. Concept-wise, as the only mage in the team, she understands the spirit world better than the others, and is the only one who can control people and make illusions, and probably the only one who can help solve the magical part of this mystery mission. I didn't get the impression you imagined her as a combat mage nor a healer nor a spirit shaman type, let me know if I was mistaken on that count. One way you could integrate Missy more into the setting is to visualise that she had been to Japan before, and can draw inspiration from Japanese mythology and mysticism.
Let me know if what I've said above helps and if you have new ideas that can connect you back. Otherwise I'll respect your break, and move Domino to join up with the rest of the team.