Executing Ace way sooner is really the biggest change I’d make, you don’t need to give the enemy a chance to come to their friend’s rescue if your intent is to send a message.
Other than that, I’m not sure what else Sengoku could have done...if the Marines had lost, there would be way more questions about his methods, but outside of Luffy’s interruption, seems like he got everything he was aiming for with his plan. Maybe give the admirals more leeway in what they could do as
@Finalbeta mentioned, seemed like they were holding back during a time in which restraint is an unnecessary handicap.