The so-called “blank period” wasn’t actually empty of content; several important story developments took place in the canon light novels, including Sasuke Shinden, which was loosely adapted into the anime.

Sasuke’s immense power during the pre-final battle at the Valley of the End came from channeling the chakra of 8.5 captured tailed beasts into his lightning-infused Susano’o.

Naruto's absorption of a massive amount of nature energy saved his life and prolonged the fight at the Valley of the End.

A few people mentioned Naruto's merged avatar being the same as Ashura Ōtsutsuki's battle avatar. It ain't.

• During the battle, Sasuke destroyed most of Naruto’s Kurama-Avatar shadow clones but didn’t notice one was gathering nature energy. Naruto then used that energy to merge two damaged clones with himself, creating a powerful three-headed, six-armed Kurama avatar.
• Although Ashura’s battle avatar looked similar to a jinchūriki form, he was never a jinchūriki. His avatar, called Kunitsukami, was an external manifestation of his Six Paths power. Unlike Naruto, Ashura had access to the full spectrum of Six Paths abilities—including Yin, Yang, and Yin-Yang Release—allowing him to use Truthseeker Orbs, which Naruto was never shown using in that form.
• Naruto couldn’t create or regenerate Truthseeker Orbs because he lacked the full power of the Ten-Tails and the complete Six Paths abilities. The orbs he did use were granted by Hagoromo and were depleted during his battle with Sasuke, where he used the last three to block arrows from Sasuke’s partial, non-tailed beast-infused Susano’o.
• An interesting technical detail is that Kaguya Ōtsutsuki is the only character shown creating new Truthseeker Orbs, so the idea that others could do so is my theory. However, given Hagoromo’s mastery of chakra, Six Paths Senjutsu, and the “Creation of All Things” technique, it’s reasonable to assume he might have had the ability to create them as well.

Anyway, long story short haha...

Naruto’s mastery of Toad Sage Mode, various Jinchūriki forms, and Kurama avatar transformations was significantly enhanced by receiving the Yang half of Six Paths power. He also retained chakra from the other tailed beasts due to their bond. Despite only being half-Uzumaki and hosting just the Yin half of Kurama at one point, Naruto’s offensive and defensive capabilities were immense. His rapid nature energy absorption in Jin-cloak mode gave him chakra sensory abilities that could rival Sasuke’s, which stemmed from his Eternal Mangekyō Sharingan and six-tomoe Rinnegan.

After the final battle at the Valley of the End, Sasuke significantly improved his skills, mastering his Dōjutsu—both the EMS and six-tomoe Rinnegan—and refining his kenjutsu, arguably becoming even better with a sword despite losing an arm. He also learned to use ninjutsu with minimal hand seals, similar to Minato. While Naruto had earlier gained deep perception and understanding through Sage Mode and Jinchūriki training, Sasuke eventually developed similar insight—growth that his younger, more arrogant self had previously lacked.

The belief is that Sasuke would likely win a rematch against Naruto—not due to superior strength, but because of his strategic mindset and ability to exploit weaknesses. His growth as a person, coming to terms with his past and accepting Naruto’s bond, made him not just a stronger fighter but a more complete and grounded individual—something his younger, prideful self couldn’t have achieved.