The truth is, that Itachi spammed his MS probably more excessively in his fight against Sasuke than Sasuke against Danzo. If you only consider Amaterasu, Itachi did create enough flames to set on fire half of the forest surrounding the Uchiha palace with it. This is a considerable feat if one keeps in mind that all of those flames consist out of Chakra. Usually, only a small amount of flames is created with every usage [
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]. Beyond that, he used Tsukuyomi
and endured its breaking, what put another strain on his eyes. Deleting the flames of Amaterasu, what Itachi also did, traceably strains your eyes as well [
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]. Ultimately, he kept up his Susanoo for a considerable time. All that while at his illnesses peak and near blindness, staying alive only due to medication.
And as Blaze said, not even then, Itachi has to refer on his Mangekyo so excessively.