Actually if you are invested in the main quest, there are plenty of loot bumps along the way that will make the rest of the game easier which explains why I never really encountered a single ammo shortage in my first playthrough.
Anyway, even if you don't want to advance the main quest, the longer you delay visiting DIamond City, the more you will feel the scarcity of resources since you cannot dump unwanted loot to convert them to bullets as easily. Not to mention the majority of EXP boosts comes from quest completions which Diamond City has plenty of.
Is it possible to just migrate the settlers to a single place? I often don't see the point of splitting up the settlers in smaller numbers across larger number of settlements, only to be constantly reminded that I need to provide them with defenses and other time consuming upgrades.