@Hellos: I am sorry for the wall of text, I did not realise how long it was till I was done. No doubt there are spelling mistakes in here somewhere, I have found and correct what I could see so I hope that is fine.
It is a well known fact that Itachi had a very small chakra reserve, given that he had to deactivate his sharingan entirely after using 1-2 of the ocular powers of his Mangekyo in part one of the series
Despite his mastery of the Mangekyō Sharingan, Itachi suffered greatly from its side effects. Each of the techniques bestowed by the Mangekyō Sharingan required vast amounts of chakra to perform, and they left Itachi fatigued afterwards, to the point of requiring him to deactivate his Sharingan altogether, something that he did not usually need, three uses of which seem to have been enough to bring him to this in Part I.
Pulled that off the wiki. Given his health in part 2 during his fight with Sasuke, it was clear that he is even weaker now than when he was in part 1. The reason he died was from over exertion with his Mangekyo, Susanoo put far too much strain on him by the end, and he succumbed to Chakra Exhaustion, and his illness did not help this fact. You also need to remember that by the time he used Susanoo his Mangekyo had lost most of its design (Meaning that he was almost completely blind), and that he used Tsukuyomi, followed by multiple Fire Release Techniques and Clones, not to mention Amaterasu 2-3 times and then finishing it off with a full blown Susanoo. He was more or less out of chakra by the time Susanoo was summoned (But he did keep enough to use a fuinjutsu on Sasuke), He kept it up, and himself alive by sheer willpower alone and the pain he was feeling by the end would have been more than enough to kill him had he given in.
While he did plan on dying going into the fight, He planed to kill himself rather than have sasuke have the burden
Despite his impressive abilities, he never underestimates his opponents, given his praise of Jiraiya, saying the best he could do against the Sannin was a stalemate. The only thing that Itachi was lacking was his chakra reserves, which were below average, presumably due to his fatal illness.
Another part pulled from the wiki in testament to his chakra. If anything that makes him even more impressive given that he could not just spam his attacks, he had to be smart and not end up killing himself.
Another thing to remember is that in the Kazekage Arc Itachi's clone had 30% of his Chakra and yet was unable to use any of his Mangekyo's jutsu (Let alone activate it), Could be he didn't want to use them, or that he couldn't at all... Just food for thought.
Another is that after using Amaterasu to break out of Jiraiya's Toad Mouth Bind summon he had to deactivate his sharingan entirely because it put too much strain on his chakra and eyes.
But everyone is entitled to their own opinion, and I shall respect yours. I feel there is more evidence in regards to his depleted chakra reserves and strain on his body, than to him falling from his illness, and I believe I made a good argument in regards to it.
Due to having much larger chakra reserve than his elder brother, Sasuke can use the Mangekyō Sharingan techniques more than three times a day. While Itachi was critically exhausted after using Tsukuyomi and Amaterasu once each, and then Susanoo as a last resort technique that ultimately led to his death, Sasuke could maintain Susanoo for extended periods of time, on top of using the Amaterasu and Tsukuyomi several times a day along with some other chakra-consuming techniques like the Chidori. Regardless, like Itachi, using the Mangekyō Sharingan puts a great strain on Sasuke's body and eyes.
Pulled that off of the wiki too, Thought it added to the Chakra Reserve situation.
Now in testament to Sasukes Chakra to solidify my point. (I would remove the links like the last one but there are too many, and it is a hassel with this laptop)
Sasuke is shown possessing a high amount of chakra. At the age of 7, he already had enough chakra to use his clan's signature fire release technique, something Kakashi was very surprised, even when Sasuke performed it at age 12, as such techniques usually require more experience and development in chakra. In Part II, he was able to summon Manda, a high level summon, while in a weakened state to protect himself from Deidara's suicide-bomb technique (though this may be due to borrowing on Orochimaru's chakra). Sasuke was also able to use many highly chakra-consuming techniques several times a day, exampled by his reckless usage of all three of his Mangekyō Sharingan techniques against Danzō and the other Kage multiple times, among other techniques like the Chidori. Quality-wise, Sasuke's chakra is noted to be very foul and powerful. When Sasuke first arrived at the Land of Sound and gave Kabuto a frightening glare, both he and Orochimaru took note of Sasuke's exceptional chakra. Even Kurama, the Nine-Tails points out that Sasuke has the same foul chakra as Madara's, whose chakra was particularly strong even amongst the Uchiha. Karin has also mentioned that Sasuke's chakra, as he approached the point of manifesting Susanoo, had become even darker and colder than the corrupting influence of the Cursed Seal of Heaven in its second stage; it has since continued to grow even more potent along with the development of Susanoo.
As you can see he has always had a larger supply of chakra when compaired to his brother, which also made him more reckless with his attacks. One reason why his Mangekyo only lasted a week or two before he was in a near blind state (Time-scalling the period when he recieved it to when he first noticed his eyesight starting to go, purely speculation of course), Whereas Itachi kept his alive for many years because he did not use his techniques so recklessly due to his low Chakra and prefered to not use them at all unless he had no other choice.
Hence why Sasuke could spam his techniques, and Itachi could not. I guess you could say Itachi prefered to use his skill over his power, where Sasuke would be the anti-pole of that using his power rather than his skill. The attack on the 5 kages would be a prime example of this, where he used every technique the mangekyo had within the first few minutes of the fight which left him near blind, rather than assess their power and attack accordingly.
Now I am not saying that Sasuke is weak or unskilled, I am saying that he is very agressive and not very paitent with his attacks and would more than likely trade attacks blow for blow, whereas Itachi would sit back and avoid attacks to wait for an opening to strike.
Now I can understand an argument of him dying as a result of the strain of the use of ocular jutsus with the disease, not so much that he simply ran out of chakra and died. Since his resurrection he's been coasting off his ocular abilities just as easily as EMS Sasuke can, this is the best example of Itachi at peak since he no longer as an issue with an illness that is literally killing him. And it isn't like Sasuke's spam of the techniques didn't drain his chakra to boot, he was nearly out while fighting at the Kage Summit.
What killed him in the end was the strain he put on himself and the chakra he had used up, but the illness did not help this. So we are both right in one way or another (Depends on how you would rather look at it). Edo Tensei would negate the cost of Chakra for jutsu meaning he could freely spam them (Its the only logical infrence given that those revived never tire, never need to eat, never need to rest, and are immortal) with no adverse side effects. And as I said before in regards to Sasukes Chakra, he has a large reserve and can afford to waste it like that now, given he no longer has the side effect the ocular powers bring with them.
Itachi has had the illness since he was a child (Which one could atribute to his low chakra) so him at peak would be him at his best fighting form with a like new Mangekyo. Best guess for this would be around the time he killed off the Uchiha Clan, or in the years that followed.
A healthy Itachi is hard to scale since we never seen him go all out until his battle royal with Sasuke, So all we can guess is that would be him at his peak but in tip-top shape and healthy eyes, with a regressed form of his illness.
The EMS only grants permanent Healthy Eyesight and immunity from the effects of its subsequent jutsu, such as strain on the body and organs, and bleeding from the eyes or mouth. (Izanagi and possibly Izanami are the only 2 exceptions to this I believe) And also increasing the overall power of said jutsu.