What causes the Susano to morph into more advanced stages
The hatred part was more of an exaggeration,which the Uchihas love to do(like Itachi and Obito).If that really was the case,how did Itachi become so powerful?He wasn't consumed by hatred.

Sasuke was only discovering and learning the powers of his Mangekyou Sharingan,which we know takes time.Although it is true that his negative emotions aka hatred forced him to use more chakra for his Sussano,thus giving him the complete "half" form.

After learning to activate this version,Sasuke never really needed to become that enraged before using it.

Most of the discoveries of the Mangekyou were by chance,just like Itachi did(Amaterasu Vs the fourth Mizukage)."Hatred" has nothing to do with it(only needed for activating the Mangekyou for the first time of course).