You should know better that the triangle theories don't work. Brook was landing shots because he was much faster. Brook was scoring well vs. Spence until he got broken down. He had the natural speed advantage of a 147 lb'er, but once GGG negated speed by walking through his punches it was over.
In regards to Canelo and Jacobs, both dudes have fire in their hands that BJS doesn't have. GGG isn't a dummy, he will box from the outside if he respects the fire coming back.
To me the biggest question of a GGG-BJS fight now is if BJS power can keep GGG honest. From a distance I think the fight is a lot closer than it would have been a few years ago, BJS could do some good work from the outside, but if his power can't push GGG back he is in deep trouble.