I could have missed it but I didn’t notice anyone suggesting you check your consistency of neck tension. I recently had a .300 Norma that I put 300+ rounds through trying to get it to shoot 1/4 moa which I knew it was capable of. I never actually checked the mandrel I was using just assuming it...
With my experience with the 156 your bc is off. I have a verified G7 BC of .330. The .347 they post on the box is way high in my opinion. Not saying this will totally fix your problem but may get you closer.