Nope, He doesn't have low chakra reserves.. he has low stamina..
As a uchiha , he was naturally born with higher chakra reserves than the average ninja..
Stamina is chakra . No he had average chakra reserves but powerful chakra .
Stamina is one of the foundations of chakra, and so is mental energy.. something uchihas are born with high amounts of..
He has above average chakra reserves, as uchihas are born capabile of using katon at extremely low ages.. Itachi mastered his before sasuke.. and kakashi stated genins aren't normally supposed to have enough chakra to do such a jutsu.. this already puts itachi above it temrs of AVERAGE chakra reserves.. does this mean he can last long in a fight? no, he's dying from his sickness and was basically on death's door
Stamina is chakra . No he had average chakra reserves but powerful chakra .
Stamina being the foundation of chakra means that having low stamina, equates to only being able to create a very low amount of chakra compared to evryone else. His reserves might be as big as any average shinobi, but his low stamina keeps him from completely filling up his chakra reserves all at once.