>>50254240In Isle of Armor, Pokemon and trainers will be a very low level before you beat the champion. In post-game, they all jump up to level 60+. So the only options are to either do everything immediately, or do everything after beating the main story.
The only "drawback" of leaving Isle of Armor until post-game is that you receive the main "legendary" in Isle of Armor at level 10, regardless of your story progression. To evolve it and complete the Isle of Armor story, you need to clear a tower where you have to beat all the trainers using nothing but that Pokemon. Before beating the champion, the max level of the trainer Pokemon is 30 but after beating the champion, the max level Pokemon is level 70. So you will need to do a bit of grinding to stand a chance in post-game (although at that point, grinding is pretty easy since you can find wild Chansey and Lucky Eggs).
In Crown Tundra, everything starts at level 60+ regardless of your progress through the story. So you're best off leaving that as post-game as well.
