You get a perk which allows you to send trade caravans between settlements....

Holy crap good luck spinning THOSE plates!

I'm not expecting this because most games just don't want to let you "get away with" that, they have to keep throwing trouble at you. I'm expecting even if we try to put in the time and effort things like limits on the number of defenses or arbitrary ramping up of the level/strength/quantity of enemies will ensure your settlement is never safe.