This is getting ridiculous. Roman and Persian soldiers, outnumbered some three to one, manage to not only attain a decisive victory, but do so at the cost of a mere 8,000 dead after literal years of campaigning with barely any defeats or mutinies? I’m not buying it. You’re telling me nobody in the Awadh army tried to rally their soldiers? That the entirety of the army on the frontline suddenly forgot how to count and literally see that they outnumber the enemy? That Chandragupta doesn’t take advantage of weakened Romano-Persian lines to break through (or that attacks already under way don’t do so)? That the Romano-Persians manage to create a cauldron around the Awadhi forces, who by your own admission still have a battle-ready force in the rear, the perfect area to mount a counterattack not clogged by soldiers, and get away with it?
I’m sorry, but this is straying from skilled leaders defeat peer opponents to my generals are better than Alexander and Hannibal and can’t lose. Seriously, this is a more lopsided victory than Austerlitz (a battle between roughly equal forces), as devastating as Guagamela (which only took place after the Persian army had been defeated twice already and Alexander had taken the Mediterranean coast, lowering morale and army troop quality), and even riskier than Cannae (where Hannibal was only outnumbered by about 10% or so).
I can accept that morale shattering would be a devastating effect, but not across the entire army simultaneously, let alone on the flank opposite of the problems. Besides, like I mentioned earlier, breaking morale doesn’t mean it stays broken. William the Conqueror managed to rally his men after they began to flee the field and came back to win a smashing victory at Hastings. And I think you’re severely overestimating the terror of heavy cavalry. They’ve been a known factor for literal millennia, I honestly don’t think Awadhi soldiers would suddenly forget that they exist and treat them as unknown monsters as soon as they show up. Not to mention that by this point gunpowder had significantly reduced their viability and the amount of armor they wore by this point OTL. I highly doubt the Romans have decided to ignore the fact that armor plating has decreasing yields when they were fighting for their lives a mere ten years earlier and needed every edge they could get. A single volley from the Awadhi would send more than a few cavalrymen tumbling to the ground, weakening any morale effect the charge may have.
If this victory is for story reasons and not necessarily what’s realistic, fine, but I am having a very hard time suspending my disbelief at the increasingly long string of Roman victories with only one real defeat in the last generation (and that one was diplomatic, not on the battlefield where the Romans apparently have the best soldiers and everybody else just has to let the Romans win).