I'll add that Isle of Armor tends to be a bit more fun when you do it as soon as possible, even if you end up overleveling for the main story just a little bit. The early reports were correct when they said it "scales", just it only scales to >=60 after beating the game, if you do it as soon as you reach the wild area it ranges from like 12-30. But yeah, a certain fight is a lot more fun when you're both around 20-25 and a certain tower becomes actually challenging if your Pokemon is underleveled, really gotta think with items there.
Also while you technically can do parts of Crown Tundra early, I wouldn't recommend it because having a level 70 Suicune with your OT before the first badge isn't as fun as it sounds.