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
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.