Sakura didn't need to train to gain her perfect chakra control. She started to fill the byakugou seal at the end of part 1, when she started to train with Tsuande. The proof is in the puddin.
It took Sakura 3 years.
The timeskip was over 2 years, When she complete the seal it was the 3 years later, Near Naruto's birthday. This next scan is about to be the kicker. Hold onto you knickers.
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Read that sugar plum, Tsuande says their chakra control must already be refined to perfection.
But what folks theres more, sharringan need to be activated with chaka hun,
Being a hyuga fan you should know this, this is why they use the handseals.
Sarada didn't need to use the handseal to reactivate her in the manga.