Check this:
https://forums.totalwar.org/vb/showthread.php?t=103172 thread out if you haven't read it already. It is a superb, detailed and illustrated guide to using cavalry.
Never try to run through a unit after charging it. Especially if that unit is in a rather deep formation. Cavalry is most susceptible when simply running through a unit. You will suffer huge losses if you do. Charge the enemy unit and pull out in the direction you charged from immediately after the charge. That way the horsemen do not become entangled. Repeat this until the enemy unit routs. Forget about the charge bonus, all you need is the initial impact. Forget about the hole in the line too. There are elephants and scythed chariots for that in EB/RTW. Only in real life can cavalry make a whole by running through a unit and only in real life is the wedge formation effective. One needs to understand that what is true in real life is not necessarily so in a computer game.