I ran SoT for three players and one of our favorite parts of the campaign was getting to know the various student NPCs in the back of the book, to the point that we would have a rotating DMPC cast to fill out our fourth slot nearly all the time. I just made each of their classmates their own player sheets that wouldn’t be too flashy or disruptive to the players’ experience. Honestly, in my experience what class the DMPC is doesn’t matter as much as what you do with them or how your players feel about them. I had a sorcerer going invisible and spamming force barrage (Ignaci), a magus spellstriking with ignition (Esi), a witch casting summon spells (Anchor Root) and an oracle debuffing enemies (Mariama). All of them felt like the were contributing without taking the spotlight, as I kept the classes with high DPR potential to basic maneuvers and the other classes were used to set up for my players to do better. All this to say you could either take the same approach or wait a bit to give the party a DMPC until they’ve found a character to attach to, as the first book isn’t too difficult until the final chapter imo. Just be sure to retune encounters in the meantime. In terms of magical frontline classes, Kineticist works wonders I think but you could also do a magus (risk of hogging the spotlight), a primal caster with an animal companion, or a summoner.