Easy to fix this, they should reduce number of cannons on galleys, especially on galiots, chebecks and archipelago frigates but increase the bonus cannons in internal seas from 2x to 3x.i am worry about this change perhaps you are making galleys to strong. heavy ships use 3 combat width, so now 1 havy ship is ecual to 6 galleys (use to be 1 to 3)
i will use the current stats of the ships in an open sea battle
In early game a Early Carrack (heavy ship) has 40 cannons and 20 ship hull, when a Galley has 12 cannons and 8 ship hull. Adjusted to the combat width, cannons: 12x6=72(before 36) vs 40
Ship hull: 48 (before 24) vs 20.
In late game a Threedecker has 120 cannons and 60 ship hull, when Archipelago Frigate (Galley) has 36 cannons and 24 ship hull. Adjusted to the combat width, cannons: 36x6=216(before 108) vs 120
Ship hull: 144 (before 72) vs 60.
In general you can stack more galley combat ability than havy ship combat ability and galleys are sustancialy cheeper than havy ships so you can go over force limit with littel penalty.
So why galleys sees to be so good but they aren't? Moral, when a ship is sunk all the fleet recives a moral damage. so in big battles galleys use to sunk 1.5-2 per 1 heavy at the start but when the battle contineu the recerves of the side with galleys join the battlel with the double of moral hits surrending before so the ratio start to become 2,3,4,5,6... per heavy ship. this change in emperator when it was intruduce the disengache mecanic, and you can still avoid the moral recerve damage as simple as reinforcing the battle slowly.
So in recap. I am worry about change the combat beacause essencialy you double the galleys stats. The havy ships will became useless even in open sea battles.
I imagin this will happen seeing only the numbers but perhapps in your test for some reason that i am not considering don't happen. i hoppe i am wrong or you are planing to introduce more changes?