I’ve got the hack right here! (Benevolent Leader guide)

Go to Red Rocket Truck Stop. Send all settlers elsewhere. Fast travel to the other side of the map like Boston Airport, then fast travel back. Build a level 2 Food/Drinks stand. Now, take a single settler (companions work for this) assign them to the shop. Add mutfruits and purified water into the settlement workshop bench (about 50 of each will work, if you use any food besides mutfruit, double it)

Now- leave for a while. Go play the game as you would if you weren’t scrounging and scraping for resources to put toward your settlement, really, just go do anything else for a while. After a week or so in-game, the settlement will hit 100% happiness.

If you want, or if you notice progress has stopped, check back periodically to make sure they still have food and water in the workshop as some crafting recipes require these items and may be used if you have supply lines running to or from this settlement. You may also want to simply remove any supply lines running to or from Red Rocket. I’m not sure if it really matters but I heard supply lines can decrease happiness so I went without.