A system of traits that caps your potencies when you sync every 10 levels I think would be the way it has to be done in this game.
For example, at Lv70, PLD has the Holy Spirit combo through Requiescat at 600 potency each. If you want to use the whole Confiteor combo at that level, you'd need a trait to cap that combo at 600 potency. Atonement combo on the other hand could be capped at ~260, a rough average of your 1-2-3 combo.
That's the kind of trait finessing I think you'd need to use to make this work.
EDIT: Also, it's been discussed to death because the higher the level cap gets, the worse the level sync experience feels.