Take a social approach to war and culture
Wars aren't just fought by soldiers, its waged by an extensive support network that eventually leads to the people of a faction.
There are a few unintended consequences that I am going to try to address.
First, by all intents and purposes an army of Rome that you described should be able to blitz its way across any foreign land with relative ease. During the Civil War Sherman and Grant marched great distances across the battlefield and covered a significant portion of land, and this was in the CSA where they used a different gauge for railway compared to the Union. Even the German army of WW2 which is often erroneously hailed as a mechanized monster relied heavily on horse and infantry trudging through mud (The armored spearhead of Blitzkrieg tends to overshadow the infantry following behind).
Essentially a foot mobile fighting force cannot be counted out and are more than capable of mounting a quick campaign across enemy territory. This is compounded by the fact that Rome's enemies aren't close to a peer threat or even capable of establishing a modern guerilla style campaign. The level of dehumanization required to fight against this type of enemy probably doesn't need to be as high as you think it is if you can quickly wage your campaign and be over with it.
An emotionless and robotic fighting force doesn't mean that said fighting force is the most competent or best at carrying out orders. A sociological approach to dehumanization would probably work better than technology. The shared ideology shared by a group can be very powerful. The Nazis didn't need technological solutions to dehumanize the Jewish and Slavic people. The Japanese didn't need technological solutions to dehumanize the Chinese and Koreans.
Really what you have is a Manifest Destiny situation on your hand. Where the US parallels Rome in your situation. Cultural attitudes towards westward expansion trumped Native American claims to land. This was reflected even in the military.
A cultural expectation that promotes expansion over the lives of any foreign inhabitants would create a self-feeding cycle where your population both pushes war/armed conflict while at the same time continually demonizing outsiders. Given that Rome is essentially isolated in this setting, you don't have the free flow of outside information to bring forth competing ideas. You have a vacuum in which not only the government can heavily push forth its own ideas, but external influence is nonexistent. The government would have its people doing its own bidding by culturally spreading the propaganda themselves. The people themselves would be able to reject home grown ideas that run counterproductive to the social narrative. Things like Feminism and Abolition didn't just gain popularity overnight and become mainstream. It was a slow and grueling process that had multiple watershed moments across it. Had a government and society at large intervened more heavily against such movements, it's unlikely they would have become mainstream. In a similar vein, during the Cold War there were communist groups in America but being outed as one or being accused of one could end one's career especially during the 50s (oversimplifying a lot obviously).
Dehumanizing only your soldiers to the point of emotional neglect and a total lack of emotional maturity leads to massive problems reintegrating into the population once they come back from their tour or are injured. If there is a massive cultural divide between the soldiers and the civilian populace you are going to foster a major "us vs them" mentality. Only in this case it would be more extreme than what is going on currently. Having a populace on the same page across the board about its views on the "enemy" would make reintegration back into society easier.
Lastly, dehumanizing Rome's enemies does not mean that Rome is no longer capable of being over the top bad if that's what you want. One of the things that we learned from the Nazis and the multiple ethnic conflicts during the fall of Yugoslavia is that culture plays a huge role in taking seemingly normal men and then having those same men commit heinous crimes against humanity. If you create a strong "us vs them" mentality, then it creates the scenarios and conditions for war crimes and brutality. At the same time, it creates a homogenous cultural expectation and view of war and the enemy. In this way your Roman soldiers follow orders to the letter and are "mindless" in the sense that they willingly follow orders because they have been socially conditioned to. That the war and deaths of their enemies is acceptable. The Nazis fought to the bitter end in WW2. Even when the Red Army surrounded Berlin and the Reichstag, the Nazis fought to the bitter end.
From a narrative point of view, if you are writing and framing it any way not directly from the Roman point of view, you can argue that as a culture Rome is more than fine and even encourages brutality and horrible treatment of its enemies in war. That as a society and even all the way down to the squad level, they are eager and ready to commit a brutal land campaign. If you're taking it from the Roman perspective, then you have a lot more room to explore their psyche towards war.
Dehumanizing an abstract enemy is far easier than dehumanizing your own people. Dehumanizing your enemy, while working your own people through the propaganda mill will result in a brutal campaign from one side if you want. The major downside is that it is highly unlikely that you will change this society unless you completely topple it. Rome's enemies aren't going to be able to change Rome's mind or its soldier's mind. You are more likely to hit a situation where the general populace rejects all notions of peace, and if things continue for long enough, they could outright deny the humanity (as in species) of their enemies. Which at that point, well the cats out of the bag.
