Hey, OP here. The abuse you experienced was terrifying. I'm really sorry that you went through it. I am glad you got out.
In my case, I feel comfortable trying to make amends with my dad because 1.) I feel pretty certain about my boundaries and 2.) I think he actually wants to improve. But he didn't want to improve until I had shown him firsthand that I could completely cut him off without looking back.
I think there are many parents who, even if are backed into a corner and given chances to improve, will not. It sounds like your stepfather is one of them. I wonder what his childhood was like, and what pains that he endured, in order to treat you so terribly.
I think what matters most for any individual person is getting ourselves to a state where we feel okay and happy about ourselves. How that process unfolds depends on the person.
Take care.