That's pretty good, obvioulsy the metal grades are dependent on a significant expansion of the types of metals that exist in the game, tin mines for Bronze production and a Steel making process to upgrade our Iron. I think Bronze and Steel might be switched in your hierarchy as Steel cannons were really not a thing until well after the colonial period, as steel was gawd-awful expensive back then.
I don't think it is necessary to make the higher cannons do less damage, this is not really logical as a stronger barrel is always able to hold more power. How about the idea of a 'safe' limit of powder in each cannon, low cannon can hold 100g units of powder (thats 120 silver for one shot mind you) safely for a little damage, but could be loaded with maybe 3 times that for higher damage potential at higher risk. Each higher tier has a higher safe load of powder but that always be overloaded to x3 the safe limit for exponentially increased damage. The highest tier walls could be damaged by the lowest cannon but only if you overload it heavily.
I'm so so sure about the elaborate set of effects on characters hit by the cannon, they sound interesting but will need balance. I like the idea of BB effects in combat, and the leadership/rally thing, allies having effects on each other humors is a good thing to foster and reward team-work.