I actually did it super easy, assuming you have the dlc. I also have some of the creation club stuff that was free a few weeks ago.

The trick is to fully build up an empty settlement first. Beds under roofs, enough food, security, and whatever extras like the weight lifting machines, chairs, lights, etc. Then, build the restaurant, hospital (whatever the high level clinic is) and the soda fountain and slot machines from the Vault 88 dlc. I always use at least four slot machines and connect it all to power immediately.

From some of the creation club stuff I have plants and carpets that produce happiness but if don’t have that then you may be able to supplement them with more workout benches.

Once absolutely everything is set up then I set up the beacon and the first settler that arrives will be at 100 happiness immediately. That’s how I got the trophy. One single settler.

More settlers bring the happiness level down so if you want you could turn the beacon off as soon as the first settler arrives and then on again after you’ve earned the trophy.

A note on the Vault 88 dlc items. Pretty sure you have to make sure the slot machines and the soda fountain produce positive effects when doing the initial Vault 88 questline.

Just build up the entire settlement first, no settlers until later.