Give cav a bonus in riding and a huge bonus for the horsearmor when they"re in formation (like in shieldwalls).
Call it formationbonus or what ever.
That would make them much stronger if they work together, charging the enemy (or archers), without buffing kiting HA.
Also this would be a much more logical way to limit the use of the heavy horses. For example, keep the riding skill req low but make them slower and give em less armor. When theyre used in formation they can use their full potential and get rewarded, if they're tanking solo, they're just another horse.