Thank you for the update! It's really helpful to hear what works for other chicks (human and poultry, lol). It sounds like your struggle was a bit difficult.
My sebright baby that was being not so nice just suddenly stopped, but the ayam cemani started picking on anyone with a black mask (SLWs and French BCM) when I added them to the mix. I too read that reaching one week old would help, and honestly, I think it did. I also added puffy bug and halographic stickers to the brooder box, along with a mirror and marbles for entertainment. For the ayam, I did try correction by gently "pecking" with my finger when she went after the masked bandits. I felt like it sort of made her afraid of my hand. Anyhow, bad behavior has ceased, and I've got 21 very well-behaved chickies living together in peace now. I didn't end up having to separate anyone.