Does Janson's ability trigger before the defender modifies dice?

Nope, the entire attack sequence is finished before you end up "after you perform an attack". Or in other words, the term "perfom an attack" includes all the steps listed in the rules as part of an attack.

No, his ability states that it occurs after the attack, which means all steps of the attack including defender's roll/modification of dice. His ability is to remove a token that would have otherwise remained after his attack.

Edit: the ninja is strong with ^^this one.

Thank you for the clarification.