Belgium
How? Sanctuary chilling at 75-82% but wont go higher. I gave them a bar, surgery center, weapons dealer, armorer, clothing shop, and the top trader thing. Them ungrateful heathens still aint happy. Im ready to give them chumps the Covenant treatment.
My garage below. Its actually the 4th and 5th floor of the crib i built. My dude living like a boss. 5 story mansion with a half court in the living room. 19 suits of armor. Finessed 2 from raiders, 2 from BoS, bought 1 frame from KLEO and found the rest. 1 full set of each, T-45, T-51, T-60, Raider Power, Raider Power II, and X-01. Just need a Atom Cats set and thats all of them. Left side are completed sets, right side are missing pieces. 1 of each color/material. Most of em still deadstock. Used my 1st full fusion core moving them all to the new spot

Dont know how to embed on here.
It's bugged. Happiness automatically goes down if you're not actually in your settlement. Look up a guide on youtube.
I used Starlight Drive for my settlement because it's very open and a good place to build. I had 16 Surgery center stores (highest level Clinics) and 4 settlers working on mutfruit (~24 food).
To count as a large settlement your size meter must be yellow.
Make sure your defence and water are high too. Defense should be 150+, water above 50 is fine.
Clinics give the most happiness, don't bother with other stores. Also
don't use Sanctuary and make sure you don't have any named settlers in your settlement. Sturges in Sanctuary for example counts as a settler but you can't command him to do anything.
You'll also need lots of decoration and furniture. Place rugs and paintings everywhere. Literally everywhere.
The amount of power you have does not matter but powered televisions help.
I recommend building a large space to place 20 beds in. Then build a few smaller houses and tv's and couches (couches are also important). Don't forget to cover as much as you can with rugs and paintings.
Once you have everything set up, gather 20 settlers and assign 16 of them to the surgery centers and 4 of them to your mutfruits (only place mutfruit, no corn etc, place 24 of them), this should give you ~24 food.
Note that you can not leave your settlement or your happiness will go down.
It took me about 5 hours to get from 50% to 100%.
Now that you have everything set up, place a bell to gather all your settlers. Go to bed and sleep for 24 hours. Go ring the bell, stand around for a few minutes with your settlers, maybe talk to a few of them, then go back to sleep for 24 hours and repeat this process.
It will take hours of doing this but eventually you'll get it. Needless to say I'm glad I finally got it lol.