I do hope you decide to do sandbox mode. It would be nice to explore and take your time.
The character's first week on this new world will still be a kinetic novel. But beyond that, the question of format is still open-ended regarding sandbox or RPG elements.
We generally consider the content up to the "duel" and the immediate aftermath to be ACT 1 essentially that establishes the introduction of the characters and setting fully before moving onwards to a different slice of life tone.