Something to remember when you travel to different regions a lot: Do Not release your pokemon.
This may seem common knowledge, but I've seen many people get arrested just here in Snowpoint the last few days for attempting this. Alolan Vulpix are extremely vulnerable to predation, and if they survive to evolve into Ninetails, they devastate the Sneasel and Weaville populations. Vanillite overpopulate because they have no natural predators but never seem to evolve, pushing Snorunt and Swinubs out of their habitat.
That's just here in Snowpoint, however. I've rescued many captive-bred and trained pokemon from the wild. These pokemon do not know how to live away from human contact. They are used to having easy access to everything they need and have been bred for decades to be docile and easy to handle. Even wild-caught pokemon can find it hard or impossible to readjust if they've lived with a trainer for a year or more, especially if they are released far from where they were captured.
If you have decided that a pokemon no longer fits your team, contact a nearby pokemon center. Every center has staff who take in unwanted pokemon and ensure they are taken care of. It's far less traumatizing and dangerous than simply releasing them, even if it takes a few extra minutes.