during the timeskip all what he did was trying to control kurama's chakra(in which he failed miserably)but he achieved a certain control over the chakra(jiraya said naruto could control up to three tails),the problem was kurama trying to take over throught the power of hate and his anger issues that made it impossible to control that chakra.so when naruto managed to seperate kurama from his chakra found controlling it much easier,and when kurama stopped being resistant the whole process became much simpler.
FRS was due to the use of shadow clones which is perfecty understandable.
SM is all about controlling another source of energy that doesn't come from someone's body(which is the thing that what naruto was trying to do his whole life kurama's chakra),so it's safe to say it will be relatively easy for him to achieve since natural energy unlike kurama's chakra won't resist him,and he'll manage to harnass it pretty easily.
but still kishimoto screwed him up half of the time,and neglect him the other half so he had to make up for him,he should made the control over kurama's chakra more progressive,from one tail,two etc to full control over that chakra,not going from no control at all to complete control.well naruto just like every non-uchiha character suffered the same neglection from kishi.
