The reason why naruto is using rasengan variants is actually quite simple & makes sense when you think about it.
A rasengan by itself is an incomplete jutsu, minato created it based on a bijuudama in an attempt to add nature manipulation with it to "complete" it.
Naruto perfected the jutsu & created an S-rank jutsu by just adding simple fuuton to it.
Now naruto has all the bijuu's chakra/KKGs, therefore instead of just adding fuuton to it he's completing the rasengan & variants with the bijuu's chakra, thus creating some of the most powerful attacks in history.
Basically a rasengan is the best attack you can make with shape manipulation, the only thing you can do to make it better is to add a nature release to it, which is what naruto is now doing with ease.
