Our Oscar is an 80lbs lab mix, 18 months old and strong as a bull moose.
I think our trainer would say "he thinks he owns you" and is trying to be the alpha dog. Sounds like you are in active training, with someone that knows a lot more than me.
But couple of things that have worked well for us.
When he is unruly leash him up to an inside doorknob. That way he is still part of the family but is confined and gives him a chance to calm down. Works very well for us.
And second Oscar could care less about a martingale collar. But he does care about a prong collar. It gets his attention, which is what you want, and lets him know we are going where I want, not where you want.
I know they are controversial, but it changed his leash pulling behavior in about 5 minutes. And maybe wishful thinking on our part but he seems to be a better dog all around. I do not jerk it or yank it, just a twist of the wrist, to exert a little pressure. Might be worth a shot.
I would be interested to hear Dr Doug's take on a prong collar