I'm with you in thinking that stuttering is at least partly due to hyperdopaminergic activity in the brain, particularly the basal ganglia (and more specifically the striatum) from what I've read. I haven't tried St. John's Worth but I'm interested if it continues helping your speech. I take magnesium which I think helps a bit, and I also have a bottle of tryptophan, which is a serotonin precursor, but I haven't really given them a try yet. The most success I've found is using a CBD tincture. The stuff I have is 12.5 mg of CBD and 0.25 mg of THC in one dose, which I take sublingually. It helps a ton with reducing the social anxiety associated with stuttering, and either indirectly increases my fluency by reducing this anxiety or directly increases my fluency by acting as a 5-HT1a receptor agonist. I think the 5-HT1a receptor may be promising for modulating the excess dopamine in our brains if that's the case, which seems pretty probable.