>>50989009only the Crown Tundra is meant to be a post-game area which is why their wild Pokemon are level 60+ even before the champion. Too bad (for some people), Peony's battle is purely optional to win and you can just go to the Dynamax Adventures an hour into the game, catch a Suicune, and run through the rest of the game effortlessly.
Isle of Armor is actually meant to be played alongside the main story, and then lazily scaled if you beat the champion.