Festival Plaza is similar to both the PSS from Gen 6, and Join Avenue from B2/W2, but there's other differences between Festival Plaza and Join Avenue, besides the PSS stuff.

---

Before I get into my setup for Festival Plaza, I'll explain how to handle the visitors.

Every time you pop online in Festival Plaza, everyone else doing so at the same time will appear in your Festival Plaza as a visitor, and will stay in your Guest List, even after you go offline.

After you go offline each time, you should go through your Guest List before you go online again, and see what everyone there (of the same gender as your character) is wearing, looking for clothing and accessories that are unavailable in your version of the game.

Additionally, you should check and see what stalls they're all offering, to see if there are any good and/or useful ones.

If they have any clothes that you want (even if they aren't sharing it with you RN), or if they are recommending a good stall, you should add them to your VIPs, so that you can page them up later, to ask for these things.

Once you have all of the clothes that you want from them, however (which may take a while), you can remove them from your VIPs, unless they also are offering a good stall.
(You shouldn't remove VIPs that offer good stalls, BTW, even if you don't want that particular stall in your Festival Plaza ATM.)

Anyway, here's my recommended setup:

-Treasure Hunt ★★*

-Any ★★★★★ fortune-teller tent

-Big Dreams ★★★★★

-Treasure Hunt ★★★★★

-Ghosts' Den ★★★★★

-Rare Kitchen ★★★★★

-Switcheroo ★★★†

This link explains why I have the Treasure Hunt ★★ in my Plaza, and also why it's in the 1st position, so that others can do this, as well.
(It's also the stall that I swap out for dye shops and fortune-teller tents as needed, similar to the Market listed in my Join Avenue setup.)

The other lottery shops are good for the chance of winning Master Balls/Gold Bottle Caps, the Ghosts' Dens are the best of the haunted houses IMO because of the PP Ups, and the Rare Kitchen helps a TON with leveling up high level Pokémon quickly.

The ★★★★★ fortune-teller tent is the most important thing here, though, and you'll basically need one to get the fortune necessary to use the main technique in this blog.

The Switcheroo ★★★ is also very important (for ensuring that you can get good Pokémon to use in the Battle Agency), but for people playing S/M, who don't have the Battle Agency in their game, I'd suggest another ★★★★★ fortune-teller tent or lottery shop, instead.

Now, here's a technique for getting the stalls that you want from Sophocles:

You see, when you are offered a stall by Sophocles is also when the next stall that he'll randomly give you is determined, so when your Festival Plaza ranks up, you should not enter the castle immediately, but rather earn more FC, until you're on the verge of ranking up again, and then save your game, before you go see what he's offering.

After you accept (or reject) the 1st stall that he offers you, go earn enough FC to rank up again, and see what his 2nd offer is.

If you don't like it, just soft-reset the game, and do the same thing again.

Every single time that you do this, the 1st stall he offers will be the exact same, but the 2nd one he offers wil be random each time, so you can keep doing this until he offers you something that you want, rather than having to repeatedly rank up your Plaza to get good stalls from him.
(Which is usually a lot slower.)

In US/UM, however, there's an even better way of doing this, and also of earning a TON of FC, by using the Battle Agency.

Winning battles at the Battle Agency gives you a TON of FC, but if you wait until you get the following fortune from a fortune-teller tent, you'll get 4× the amount of FC that you otherwise would from it:

"Your luck might improve a whole ton if you use Festival Plaza."

This can allow you to earn so much FC at once (even multiple thousand from just 1 round of battling), that it causes something interesting to occur.

You see, once you've gotten your Festival Plaza up to Rank 100, it takes 300 FC to reach the next rank each time, and it stays that way forever, but if you earn enough FC to rank up multiple times at once (which can only be accomplished in the Battle Agency, AFAIK) you only rank up once, and Sophocles only offers you 1 stall.

If you look at your Festival Plaza status screen, however, you'll notice that it says you need 0 more FC to reach the next rank, and until your Festival Plaza Rank catches up with where your FC is at, you'll rank up each time you earn even 1 FC anywhere in Festival Plaza!

This is makes that technique for getting the stalls you want from Sophocles WAY easier to do, and you don't have to avoid entering the castle all of the time this way, either!