Basically, the relationship between cavalry, infantery and ranged units should be that of rock, paper, scissor, with certain exceptions. And with balance between units of different factions being balanced by cost. One gondorian being better than one orc shouldnt be odd, but a group of orcs versus gondorians for the same value should be 50/50 in outcomes.
Cavalry should also be given an impact attack based on mount mass×speed+potential modifier. So for example, horses may have mass 1, but rohirrim horses being fastest among horses would have the highest impact damage, while boars with their tusks would get a modifier bonus, and rhinos with higher mass would get bigger impact that way. Wolves could get versatility to compensate, the ability to scale 2 high blocks without jumping, making them invaluable in rough terrain and while raiding fenced areas.
A thing that should be added is also a slow down when impacting, making mounted units vulnerable to being bogged down (dmg if mass(1)×speed(0)=0) if charging a massed infantery unit, but invaluable while attacking loose formations. And allied units shouldnt get impact damage, as that would make moving cavalry units around impossible.
Balance thoughts:
Dwarves have the best armour. That shouldnt be disputed. They deal decent damage, have good HP, but are slow and expensive.
Elves have centuries or even millenia of training, experience, and well crafted equipment. Their armours are lighter than dwarves which reduce their armour ratings but they should have increased attack speed.
Orcs rely on numbers. Their armours are mish-mash pieces they have collected, if they have any at all. And their weapons are crude. They should have lower attack and defence than standard but be cheaper to compensate.
Uruks are almost as big as humans, and they have better training than orcs and better armour, crude but thick (with a tendency for naked legs...) putting them somewhere between elven armour and gondorian.
Gondor, hardened by perpetual war, their soldiers have effective training, and good equipment. The strongest individual human infantery, though weaker and cheaper than elves.
Rohan, relying heavily on cavalry impacts while offensive and fortifications for their infantery when defensive. Their armour lags behind gondor. Mainly fighting border skirmishes agaisnt dunlendings or passing orcs, their attacks also lags behind gondor in all senses except their cavalry.
Rhûn, well armoured, about as well as gondorians, but less experienced in warfare as their neighbours are less hostile, their attacks would be slightly lower. Their cavaly have better armour than rohan, but deal less damage.
Harad, mainly clad in cloth, they are squishy. They rely on tactical advantages like moving fortresses to level the battlefield, but lacking mûmakils they ought to simply be cheaper than their gondorian counterparts.
Morwaith, living on the plains, basically naked, they are the definition of squish. But they ought to have higher unmounted movement speed, relying on kite-tactics, or baiting enemys towards their mounted damage dealers.
Taurethrim, living in jungles, they are seldom the aggressor. I suggest they should be able to treat logs as ladders, and seek elevation when fighting, climbing trees and attacking from above with ranged attacks. This would make them highly specialized, and immune versus melee units during the right circumstances.
Hobbits are naturally stealthy, able to almost instantly disappear when annoying people are about. The should probably have a ranger-esque stealth, making them invisible until they first attack, giving them the first attack on enemies.
Dale infantery should probably be right between rohan and gondor in armour and attack.
Dorwinion is a mix of dale and high elves so...