Not really experienced aggressive Jacks myself, a pal of mine had 2 and they were fine.
My MiL had a Jack x Fox terrier who was chill AF, so we had Jack crosses on our radar from the start.
He doesn't bark too much, unless he sees a fox, then the genetics kick in and he wants to fuck them up.
I've always thought quite negatively about Jack Russels because they seemed so aggressive, do you think being a cross has helped?