Once I get more than about 10 settlers, I give up on assigning anything and, since I don't really store anything at my settlements anyhow, I don't even assign beds anymore.
I figure - I need 2 settlers for security (that's 6 towers, sometimes 8 for some reason) and 6 settlers for farming (that's 36 plants - that's enough at any one settlement for me) and sometimes I assign the other 2 to scrapping tables.
I've just given up on assigning people and just dress all the men in harnasses and the women in road leathers and give them good weapons and plenty of defensive turrets and leave it at that.
They don't even recognize me as their leader or "boss" anyhow and treat me like furniture so why should I administer them so much.
Besides, I have so many settlements that I haven't developed at all (basically because I never have enough wood) and they take up so much time!
Saturdays are my administrative day so I'll be visiting all of my settlements and selling off my junk. Hopefully I can do something with Country Crossing, Finch Farms, Hangman's and the Coastal Village.