If you are to downgrade, you would either have to homebrew (don't know the specifics) or factory reset the switch and use a physical copy. If you want to do normal, pre-1.1.2 is recommended as it saves a lot of grinding. As for casual difficulty, 1.1.2 is pretty well routed and no grinding is required to beat the game. Same goes for NG+ categories since they do not use Tephra skip.