Right. That's not clear from your post. Try rosemary in the mix. I use teatree, rosemary, lavander and peppermint. I still get dandruff though. Nothing I can't live with. The BS eats away most of the dead skin, so at least I know the inside of my dreads will be clean.
I don't think that Bronners is specifically for dandruff. It is for washing only, as far as I'm aware. Try the BS/ACV mix.
The bronners is leaving a residue. You must have hard water. You should get a clarifying shampoo and then switch to another shampoo. I'm sure you've heard about this already.
Also wash with luke warm water and do a freezing rinse at the end.