Actually Tayuya's Demonic Flute is the downfall of Itachi, had Itachi been alone then he wouldn't be able to break it. The manner of that particular sound genjutsu being broken by Shikamaru in Part 1 was very different due to individual abilities, in this case Shikamaru's Shadow Imitation technique. As you can see
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when Shikamaru was caught its obvious he was paralysed. As you stated Shika only escaped by breaking his
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through his technique. Furthermore, if you read what Shikamaru says the distance of Tayuya only helped him even more.
Now Itachi could've done the same thing whilst in this
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but the level of the genjutsu seemed different and more powerful than before as both Sasuke and Itachi had no control of their chakra. This is why you can see their Susano start to deplete and fully gone by the next
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, offering no protection what so ever. It is only after Itachi and Sasuke had genjutsu on
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were they able to bring back their
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.
So the question remains, in what manner would Itachi break this genjutsu with no control of his own chakra whilst paralysed at the same time? Itachi would need an external source to break it the way Shikamaru did.
Furthermore, I'm not sure why the level of Demonic Flute was different since Shikamaru was able to use his chakra to use Shadow Imitation but the following pages show that Tayuya's sound genjutsu consists of different variations as it had different names upon comparison as seen
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and
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.