While the same cannon shows up all the time in other battles, it's pretty rare that the player stays in the range where it is effective, and at all other ranges the cannon is very mediocre.

I checked an old legendary campaign and it looks like the CSA is mostly using 10pdr parrots at the heights. Oddly, these cannon do their best damage at maximum range and units in town will be positioned near perfectly for that to occur. The fortifications don't provide any damage bonus to artillery

It doesn't change the point about artillery placement, but there is a sort of friendly fire in that all ranged attacks deal 5% of their damage to any unit within a small area. But in practice, even if you notice, it's still worth firing into melee nearly every time.

The fortifications really are a mess. Improving their bonuses helps a bit, reducing their flanking angles helps a good amount, but the really long ones are just focus fire traps that can't be saved.

Darth confirmed they were supposed to be applying but they were on to other projects by the time I found it, so it never got patched officially

I originally thought it worked as well, but when I dug into the game code(unity decompilers) to try to reduce how much damage units could deal from what was supposed to be impassable water it turned out it wasn't.

Unfortunately, there is a bug where the tooltips show a damage penalty but the actual modifiers used by the damage calculations are all 100%. So you can just leave everything in the best cover you can find.

If you don't want an overhaul mod but do want bug fixes and quality of life changes there is also a more minimal mod. It leaves the base game balance mostly unchanged and the changes it does make can mostly be turned off.
forum.bermudaclub.net/topic/25750-...

The AI is also mostly setup to be locked into its defensive positions. Start a long march and it doesn't sufficiently punish or react at all. Modding can improve this in some cases, but making the AI too aggressive just means the player defends for a while before advancing.

List of mods here forum.bermudaclub.net/topic/26600-...
There isn't a mod that increases XP gained though j&p does include modifiers that would let you. Using all possible time in battles, killing everything, fielding all recruits as soon as possible, and low losses can result in very high XP gain.

The game displays AI reinforcement reports in a very confusing way, it isn't even close to 1:1 with what the AI gets. When you wipe out 40k and see 60k reinforcements it's actually just resetting to a minimum, not giving the AI a huge boost. There are also mods that let you limit or disable this

The CSA campaign start is easier but in the mid to late game a lot of players have problems with running out men. As the union if you can make it past Malvern Hill you're given so many resources that it's hard not to win the campaign.

On top of all of this you're also given very small starting units where if you take too much damage in the initial phases you lose the ability to effectively rout units and just slowly lose to attrition.

I'm one of the devs who worked on the game and it's nice to hear that you're enjoying it in spite of its flaws. Unfortunately the publisher chose to rush the game out the door as part of closing down all of the game labs teams.