So I was wracking my brain trying to figure out the significance of Nezuko clenching her fist after kicking Tanjiro, and I don't think there's a manga answer because I read somewhere that Ufotable redid the scene and expanded it significantly to try and really pull one over on non-manga readers (it worked, fuckers).
It hit me a few days ago that Inosuke made a very similar gesture early in the entertainment district arc to show Tanjiro the rank marks on the back of their hands. Nezuko is showing Tanjiro the place where her rank mark would be if she had been formally inducted into the corps. Now the kick already makes it pretty clear where she stands on the matter but the gesture is a reminder to Tanjiro about the duty of a Demon Slayer to make sure he honors her decision and finishes off Upper Four.
Maybe.