You hit the nail on the head. Settler managment is non exsistant or might as well be.
Giving settlers an outfit that corrisponds with the task you have them on works ok for one settlement. But there are around 30 possible settlements. If you take an average of 20 settlers in each settlement your looking at finding 600 outfits to do them all. Not happening in my game. Even with mods to make clothing its not worth the hassle.
My advice is pick one or two settlements to focus on and let the rest manage themselves. I have found that if you build the things, (food, water, shelter, beds, and defenses), that they need the settlers will automatically start farming so they don't starve and hover around 70% to 80% happiness.
Right now its like the game is broken into two parts. Settlements, Sims Fallout style, and all that goes with them. Dealing with micro managing them, scavenging to get building materials, radiant settlement quests and attacks, happiness, etc. Then there is the rest of the game you can't play because your getting spammed with all the other settlement stuff and don't have time for it because your busy trying to avoid failing a kidnapping or raid and ending up with stuff destroyed and/or dead settlers.
I am currently on my third character/play through. My next one is going to be limited to just a few settlements and the rest are going to get ignored beyond what I need to advance the Minutemen quests. I am planning on using Sanctuary because of the Minutemen tie in. Greygarden for food production and the merchant Green. Starlight Drive In will have a defendable building built over the water hole with some defenses but there will be no settlers. I will use this location for water production above and beyond what Sanctuary puts out. This is my money maker. Finally I will make the Red Rocket Truck Stop my home base with no settlers allowed in. Just me and my companions. I may relocate my base to the Drive In once I unlock the perks that allow me to build workshops.
Every other settlment will be cleared and salvaged. The salvage will get taken back to my home base for building there. If there are people already living at those settlements I will move them to Sanctuary/Greygarden. If the settlers can not be moved they are on their own after I get done.
I have not decided yet, but I may set up defenses at The Slog as well. Just so I can have a place to send the Ghoul settlers that might show up at Sanctuary and Greygarden.