PI please rebalance Cavalry, look at in game unit description for ideas!

[Comes Imperii]

A part of the problem with expecting historical proportions of cavalry, is that many of the functions of cavalry are off the battlefield. Unless the game reflects those capacities in maneuver bonuses (shaving off a day or two when pursuing an army, selecting more favorable terrain, better attrition) then its hardly a surprise cavalry is under-represented.

That's absolutely true, although it could be partially resolved by making cavalry move faster on the strategic map, which is moddable.

In battle, infantry is queen and artillery is king.

This is a highly debatable proposition. It may be true for the late game but not really for the early and mid-game. In fact, in the later period of the thirty years war armies had almost as much cavalry as infantry.
That was mainly due to logistical considerations, but there are many examples of battles in that period which were won by cavalry. Infantry held the center but the outcome of the cavalry battles on the flanks almost always determined the winner.

Is it worth pointing out that the age of the cannon and rifle infantry was a big reason that cavalry's role was reduced, and its reflection here is the general weakness of cavalry passed, say, the mid-game? While even Napoleon did keep a strong cavalry in toe, their purpose wasn't to dominate the battlefield. Scouts, lead units, or for chasing down broken units; guards for the artillery, but nowhere near the beasts of the field as they were during cavalry's peak of importance.

The predominance of infantry was mainly due to circumstantial factors which made it much better in European battlefields compared to non-European ones. As such, there is no basis for asserting that infantry was absolutely superior to cavalry, as it's the case in the game.
It's funny you are using Napoleonic warfare to support your arguments about a game which spans from 1444 to 1821.