I am heavily abusing naval invasion mechanics and how they absolutely brainfuck the AI. I still almost lost this last war (liberating New Africa and taking Pennsylvania) because Great Britain's idiot fleets were set to "defend naval invasions" and wouldn't help me
The US is so vulnerable to naval invasion when they keep their capital in DC. The first thing I do is switch it to Pennsylvania or Illinois. Otherwise they're like Russia and Ingria where you can just neuter a massive superpower in a couple months.
I mean yeah it's vulnerable in general just having the capital on the coast, but the AI will like send an army to the Dixie HQ to stop an invasion (drawing it away from another front!) but if you cancel the invasion they'll redeploy to a front, giving you a couple weeks to invade unopposed as long as they don't have extra armies (which they won't if they're busy fighting the French or British). There's no penalty to canceling a naval invasion apparently, you can immediately restart it the next day
The secret to killing the USA early is to zerg them with rapid advance cavalry stacks, they only start with 10 regiments so you can crush them into dust easily. Always take Pennsylvania and New York, they're dead without them
I've been able to take them out early with all sorts of minor powers with this strat. One time I annexed or puppeted all of the USA as China by 1855
Literally why I move the capital to Moscow every time I play as the Russian empire. Used to also be because under the old military system uncored provinces never sided with factional revolts and could be used as military-industrial belts to prop your government up during a fierce Civil War that has you facing off against all cored provinces then downgrading their military bases and conscription centers all the way to irregular infantry before the civil war event fires to weaken their military power and then steamrolling them as soon as you're given the green light to do so
Now your capital never revolts so sometimes it's useful for it to have a port so allies can land to defend it.
Just have uncored coastal land connecting to your capital. Unless you have an undisputibly powerful army and/or navy to batter away all attempts to snipe your capital provinces, its better to keep as much buffer zones between it and any possible enemy attack.