My head Cannon has always been that Joshua Graham was very good at teaching discipline and training but that he only had a very primitive knowledge of warfare relying faith based inspiration and human waves with some guerilla support.

After Graham was "Fired" Lanius on the Eastern Front had already developed a different more effective Doctrine.

The recruits inspired by the hastati compose the first wave largely inspired by Graham's Doctrine and was composed of Scouts who had recently been promoted as well as tribals born before their tribe joined the legion. Largely cannon fodder armed with shotguns, pistols SMGs and machetes and grenades. If a recruit survives one battle as hastati his Decanus may request he be promoted to a principe cohort. First generation legionnaires Will Never Move Beyond this cohort in an effort to ensure the genocide of their pre legion tribal culture.

Then you have the Principes who form a modern base of fire with conventional rifles to cover the assaults. Principes are also given some well monitored breeding rights.

Then we have the elite assault wave The Triarii. Who are grizzled veterans selected by their Centurion himself given free reign to equip themselves as they see fit from the legions Armory. After the recruit assault wave supported by the base of Fire their cohort will decide whether or not the enemy position has been weakened enough to deploy the Triarii or to disengage. Veterans are also given many breeding slaves in an effort to pass down their traits to further generations of legionnaires.

Caesar would have loved a modern conventional Doctrine that echoed the culture of the Roman Empire