Okay you need stamina to produce more chakra which I understand is correct but it’s not the same thing. You already have an existing reserve before producing more chakra. Itachi cannot have low chakra if he has the reserves to utilize the mangekyo sharingan power such as Amaterasu and Susano’o. Even to maintain the mangekyo sharingan requires high reserves of chakra.
Reserves and stamina are two separate things, reserves are a set amount of chakra someone can use before dying, stamina determines how quickly the user recovers from using chakra, but in addition to that it also determines how much chakra you can control in on tech. FRS for example wouldn't be possible without Uzumaki endurance.
Kakashi ended up in hospital all the time because he didn't have the stamina to use Sharingan as a non-Uchiha even though he had the reserves to use Kamui. Him being able to spam MS later on isn't because he got an increase in reserves but rather because he got an increase in stamina.
Think of it like magic regeneration, if you recover chakra quicker than you use it then reserves are not really a handicap, it's why Nagato was able to spam his Rinnegan even after running out of chakra. Puts all his power into powering 6 bodies for two decades while simultaneously producing a city busting jutsu, runs out of chakra but proceeds to create a moon 10min later. #UzumakiGenes
Back to your original point, Itachi's feats came from his chakra potency mostly. He had the reserves for Susanoo but his chakra potency means the reserves don't run out as quickly, potency allows someone to use limited amounts of chakra with high amounts of power. Ashura had the chakra of a JuubiJin but it didn't stop an MS user from rivalling his feats and endurance.