This is all spitballing, but attempting to focus on the ideas that "short" mass combat sub-games should try to be as abstracted as possible, offer meaningful strategic choices, and take up as little time as possible to resolve.
Morale/Discipline/Formation Durability & Formation size & scale
The "durability" of formations could very well be their morale; and morale/formation durability be calculated based on the scale of the conflict. That is, if you're counting by 10's, each group of 10 personnel is a single point of moral for the unit. If you're counting by 1000's, only a group of 1000 personnel count as the amount you care about.
This would allow a formation of 10 people to grow to 1000's, and the numbers used to calculate their performance as a whole not spiraling ever upwards.
While "Big numbers" can feel like an improvement, they really don't matter when the enemy is using equally large numbers; and simply changing the scope of the minimum amount of troops you care about becomes easier.
Comparisons of Yu-Gi-Oh! creature cards with similar from Magic: the Gathering gives M:tG a decided advantage due to simplicity of combat resolution.
Formations
Right now, I'm still feeling "positions" for formations to be slightly more abstract locations on the field (yes, there are two flanks in opposing forces b/c the forces are presumably facing each other and could use their right or left, but generally you care about the primary flank (e.g. the one you overloaded in a bid to punch through/around the enemy's robust forces to strike at their vulnerable ones). With Flank, Van (or centre), and Rear.
For units to include mixed forces, you're best calculating the aggregates by the gross categories of armed forces, as well as their most notabl contributions to their side. For the most part "infantry" are a defensive unit. "Cavalry" are mobile. While "Artillery" are offensive. Units should have some degree of each: defense, mobility, and offense, but one shouldn't be surprised when statlines are
[note: these numbers are purely inventions, and would require rewriting]
Hobgoblin Legionaries: D: 6 M:3 O:2
Ogre Juggernauts: D:8 M:3 O:3
Human Phalanx: D:15 M:3 O:4
Frankish Chevaliers: D:3 M:12 O:3
Persian Manticores: D:2 M:16 O:5
Ethereal Kommando: D:3 M:30 O:6
Archers: D:5 M:2 O:5
Crossbowyers: D: 4 M:2 O:8
Manogols: D:0 M:1 O:16
Gonnes: D:6 M:1 O:18
Even then, stats could be simplified down to a units primary contribution to keep calculations streamlined. Only infantry contributes to a formations defense. Only mobile troops contribute to the formations mobility. Only offense units contribute to a formations offensive ability.
Hobgoblin Legionaries: D: 6
Ogre Juggernauts: D:8
Human Phalanx: D:15
Frankish Chevaliers: M:12
Persian Manticores: M:16
Ethereal Kommando: M:30
Archers: O:5
Crossbowyers: O:8
Manogols: O:16
Gonnes: O:18
Presumably you'll want your "flank" to be made up to mobile units, but also some Offensive and Defensive units to ensure their ability to: hurt the enemy a lot, and for the formation to survive the battle.
Presumably you'll want your "vanguard" to be made of a heavily Defensive, with Mobile and Offense units to give it some legs and teeth.
Presumably you'll want your "rear" to be heavily Offensive, with Mobile units to make the formation better at reacting to changes, and Defensive units to protect the mostly defenseless siege engines and various forms of artillery personnel.
Battle Resolution/Phases
The combat phases could be simplified/broadened to be: Command, Morale, Disengagement.
In the Command phase, both sides issue and resolve the results of commands given to their formations.
The Flank formation are generally used as "extraordinary" forces, and are generally used with the aim to upset the enemies battle plan. They can elect to:
-Attacks the enemy Rear (offense vs mobility)
-Intercept enemy Flank (mobility vs defense)
-Counterattack enemy Vanguard (defense vs mobility)
The Vangaurd formation is generally the largest in order to ensure the stability of the force over the course of the battle, and can elect to:
-Advance on Enemy (Mobility vs Defense);
-Entrench Position (large penalty to Mobility, bonus to Defense vs enemy Vangaurd and Rear formations, but not Flank formations)
-Defensive Arrangement (slight penalty to mobility, bonus to defense, but only vs Flank formations & Rear formations)
The Rear formation is meant to contribute meaningfully, but indirectly, and can elect to:
-Target enemy Flank (Offense vs Mobility)
-Target enemy Vangaurd (Offense vs Defense)
-Target enemy Rear (Offense vs (Offense or Defense (?)))
Ideally it feels like 3 bouts of Rock-Paper-Scissors, but your formations actions might get bypassed entirely instead of merely beaten.
Amount of actions to resolve
Attempting to reduce the actions to one per formation per side for "action resolution". So that's 6 actions for two sides.
With Morale being calculated based off of the previous 6 results to see which formations in both lines are threatened with breaking (I'm envisioning as little as 10% of formation being lost as casualties to threaten discipline; but I'm willing to go for a low as 5% casualties to threaten a formations morale).
Finally, the Disengagement phase should be when most casualties (as much as 90% of casualties) should occur. Calculating who withdraws should be based on how many formations were forced to make morale checks (xor failed such checks). With armies who have one formation fail their morale check leading to the army making a withdrawl. Armies who have two formations fail in morale leads to an army retreat. While all three formations failing their morale checks leads to an armies rout.
The difference between an withdrawl, a retreat, and a rout should be based on how many of your overall formations failed in their discipline in the face of faltering morale. A withdrawal should only suffers as badly as it did when it first was commanded. A retreat should suffer about twice as badly as an ordered withdrawl. While a Rout should suffer 10x the casualties it did when it followed its first command.
The third phase might be more of a tabulation of casualties, than making any sort of decisions for personnel to fulfill.
So, trying to pare down the dice rolls down to 12; 6 for actions; (up to) 6 to determine if a formation stays or withdraws. With the final stage calculating how badly the withdrawl goes for the defeated side.
Which leaves room for additional turns or mechanics to be added; like espionage/sabotage, engineering/demolition; supply/looting.
Ideally, players are focused on the Rock-Paper-Scissors mechanics to make them feel like they have made strategic choices. With the understanding that a small mistake can snowball into a rout if they make a mistake in how they deploy their forces preventing savvy players from risky maneuvers which would cost more than they could achieve, and encourage committing heavily to the strategy they plan on using this battle.
Specialization of units
Aspects like "better armour" could lead to personnel gaining a bit of an edge for their formation of personnel (however, adding 10 spider-steel suits to otherwise leather armoured troops matter more when the formations are 5-10 personnel, and matter much less when formations are 100-1000 personnel). While "different weapons" could allow for the conversion of one type of personnel into an other type. Personnel who are flyers/teleporters/invisible don't necessarily need new mechanics solely for their abilities, and simply giving them massive mobility/offense/defense bonuses are likely enough to represent their abilities.
Even an invisible Hobbit can get inadvertently knocked out in a battle where five armies are raging against each other.