Running out of energy in single target is completely normal, intended design of Assassination. As a burst spec our damage is mainly loaded into Deathmark/Shiv and massive Envenoms through stacking multiple nature damage buffs such as Lingering Darkness, Shiv, Darkest Night, Amplifying Poison, and more. This comes with the natural downside of our damage suffering outside of these windows, in part due to the designed lack of energy slowing us down.
Because of this, there is only one minor (0.3%) min-max with energy, which is pooling 3-4 seconds into an upcoming Kingsbane. Ideally this would put you at around 50-60% energy before you enter your Kingsbane, which is more than enough energy alongside thistle tea. Outside of that, spam Envenom and just accept that you will run out of energy. There is nothing you can do to not run out of energy. Playing around Envenom uptime also is not relevant anymore, as Twist the knife allows for two stacks at a time, which reduces wasted uptime dramatically. Without twist the knife, wasting uptime is unavoidable.