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
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.