Breaking a shield seems highly unlikely, considering you only hit the shield with a melee weapon if they are holding it up, and they can't hold it up while attacking. The only reason to block would be if you screwed up your thrust and still had time to pull up the shield.
I'd also recommend figuring out a combination of factions which have access to identical lances and at least similar strength heavy armor.
Although one recommendation I'd make is instead of rounds, just repeatedly charge until one rider is dismounted (killed), with a horse kill causing a disqualification. Reason being that it is a joust, not a foot duel, and since the horse's nose is the closest available target, anybody decent on foot combat will just aim for the horse and get to fight everyone on foot.