Btw, trying to regulate the amount of cav by buffing archers is a terrible idea. There's something worse than mass cav, and it's mass ranged.
Furthermore, archers should not be the one killing cav, rather the opposite actually. Currently in the game though, melee are much more appealing targets for cav, as they have many more reasons to be unaware than archers.
I think a change that wouldn't affect other classes but still make cavalry harder to play would be punishing collisions further than just an horse rear. The rearing animation already puts the rider in significant danger, but it really feels unrealistic, even more so when one horse blocks another.
All collisions should inflict damage to the horse depending on speed and additionally dismount the rider above a certain threshold. The same thing should happen with polearm horse rearing.
Horse-horse collisions should hurt both horses and have a much lower damage threshold to bring down both riders. This might need some tweaking as 1h cav requires the use of horse collisions to stop and hit other riders.