I can warn you the postgame is shallow, as to be expected from every first game of a new generation. There are not many good places to train, your only trainer rematches are Morimoto, Battle Buffet and the Elite Four, the former two being only once a day. This makes getting to Lv100 hell if you want to use Hyper Training.
There's also Battle Tree and Battle Royal, but the opponents in there are absolute cheaters, even in the lowest tier mode. They come packing Z-Moves, ridiculous moves and even Mega Evolutions that can't be obtained outside of event distributions.
There is still stuff you can do in Poke Pelago, as well as completing the Pokedex. But outside of that, what I mentioned is what you'll be stuck with until the next games.