Two weeks later I had another light hitting session with Pacific Tough Gut / WC Mosquito Bite in Prestige LM Mid. It now feels more like normal natural gut / poly, not too lively/powerful but initial "toughness" is now gone. Serve was accurate and effortless, with no ill effects on my shoulder. It is good to know that stringbed helps to launch the ball in a controllable way.
Breaking the sound barrier with my forehand