Re: Cavalry rebalancing
From my gaming experience (largely with Med2) with TW titles; cavalry can be OP, as long as it is limited in numbers.
To put it in other words, in the past centuries heavy cavalry was the tank forces, litterally stomping over any other unit other than heavy cavalry itself (at least in plain open fields battles) and this remained valid unless new stronger ranged weapons became available (better and faster to reload crossbows, reliable gun weapons); the use of spears/pikes is largely overestimated: they are mostly ancients weapons and they have confronted cavalry with little results over centuries.
The key here is training, for both heavy cavalry and pikes/spears: the real heavy cavalry, those lads able to crush everything, were people training for all their lives, hence they could be OP but there should be only little of them, while light and medium cavalry should me much less effective in charges and suitable only for flanking and chasing enemies (or light archers/javelins/etc). The same is valid for spears/pikes: it needs more than a spear and a shield to make a spearmen; the petzeteri of Macedon are a good example, as those lads trained hard to do only one thing basically, be the anvil of the Macedonian armies.
So it depends on who is confronting who: having a bounch of higly trained "knights" stomping over regular lancers should be the rule, just like seeing light cavalry impaled on the pikes of hardened and experienced lads.
Backing to the technical aspect, I'm not expert with RTW, but do you have a charge bonus on the attributes? If so, you can increase it and lower down the other attributes (mostly attack, you can leave defense on higher values); alternatively, can't you simply nerf down light/med cav stats and keep the heavy's ones as they are now? is it possible to increase pool recruit replenishment with RTW?
I apologize for my silly questions, I have limited modding exp only with Med2, but I worked on unit balance as a tester, hence I can share a bit of my exp here