The obvious option would be brain implants controlling the emotions.

But then you'd preferably explain how the brain implants work, or provide the basis for it to be a written truth that brain implants are simple enough technology that the society has them available on a whim.

Hm not a terrible idea especially since I already put it where they have these specialized implants in their brains that allow for Neuro-telecommunications between each other for faster and silent communication, I don't know if there's an equivalent to this but I wanna say it's kind like codecs from metal gear. That would make sense but I already have one subject where when she was experimented on she was given an experimental implant that would release serotonin and dopamine everytime she does what she's supposed to do in an attempt to create obedience through positive reinforcement, but because she kept being ordered to kill her brain made the association and instead became literally addicted to killing which eventually resulted in instability. In addition I wanted to put it where what happened to these subjects were reversible to a degree, but that does give me an idea though, maybe an experiment to do the polar opposite where the subject has no emotions..

Plus I wanted to do a storyline where some of the subjects rebel and escape, while a vast majority are tok indoctrinated to join, so that's part of the reason why I rather keep it all more psychological than anything else

That's an interesting way to engineer the implants for the purpose, and actually much simpler and straightforward than what I'd consider necessary, I guess because I'd want a solution that rules out self-control by default. With such implants, the individuals would still have empathy and moral choices, that could create lots of issues with controlling the soldiers.

Exactly, now given in my story because they were literally raised from birth in the facility and were mentality conditioned to the point where they don't know right from wrong, not even knowing that the experiments done to them were actually pretty evil. The ones who did especially get tired of the experiments rebelled and escape with some being slightly more aware than others simply due to social darwinism. The rest however were too indoctrinated as far they were to concerned hence why they were left behind. The girl with implant becomes a psychopathic murder junkie looking for her next fix but eventually is helped by loved ones to get over her addiction. In case you haven't figured it out I like redemption stories. As for that idea of emotion of blockers I may consider that, especially since I can put it where someone had it as an experiment but it failed one way or other. Other than that I do want it to mostly be psychological on the other subjects so they can be redeemed.