The first thing that catches my eye is Porygon2. It doesn't really fit the team since everything on your team is fast. You don't have Trick room, but the set you have doesn't really get anything done. Protect is almost useless on it since it doesn't go down easily. If the intention was to Toxic-stall things, I really think there are better Pokemon for the job, and better ways to use Porygon2. Others might disagree with me on that.
The Garchomp set is not something I agree with. There is little to no reason to use a Jolly nature and a Choice Scarf. Part of the reason to use such an item on a Pokemon is so they can run a more offensive nature since they no longer need a fast nature to outspeed. The more standard Garchomps nowadays are even using Adamant natures without a Scarf. A Jolly nature is really unnecessary, as using an Adamant will only leave you getting outsped by random scarfed Kartana and Timid Scarf Lele, while the added power in Adamant will leave you picking up a few more KOs.
Also, Iron Head is inferior to Poison Jab this format, as Tapu Koko and Tapu Fini are weak to Poison, but not steel, and Bulu takes more from Poison. Using Outrage in Doubles is NOT a good idea, as you are putting the game in the hands of RNG by letting Outrage pick a random target, as opposed to Dragon Claw.
The Tapu Koko looks fine. I would recommend putting Discharge as the 4th move, since you have Garchomp on your team.
For Assault Vest Kartana, since its Special Defense is terrible, the item only works if you put defensive investment into the Kartana. Assault Vest gives a 1.5x boost to your Special Defense stat, so the more investment you put in, the more Special Defense you'll get out. If you need help coming up with a good spread for an Assault Vest Kartana, try looking up one of the many that are floating online.
Since you have Tapu Koko, I'm not sure if having Thunderbolt on your Nihilego is necessary. The next most common move is Hidden Power Ice, but I understand not using if you don't have access to that. It's also not totally necessary, since your have more then one check to Garchomp and the like. I don't have anything to jump to immediately, but look through the rest of Nihilego's movepool and see what interests you.
Overall, I would recommend switching at least one of your Pokemon on your team. Either switch out Porygon2, or switch one of your other Pokemon for a slow Pokemon and put Trick Room on Porygon2.
