I do slow oven, the up the heat for the last, maybe 10 minutes?
I tend to put the slab of skin in the oven intact for a few minutes. The heat makes is much, much easier to cut, whether with a knife or scissors. I cut the pieces to about 1.5-2 times the size I want when finished.
The air fryer has revolutionised all this though, in the the heat is very even, and dry, and rendering the fat often puffs it up. Gammon crackling is the best, without exception.