I mean storywise, you didn't miss a thing. You completed the Post -Game Story, so that is it. A couple of Ideas of what you can do.
1. Have fun at the Battle Tower. Since you can use Zacian, Zamazenta and Eternatus here, all you should have to do is EV train them. Now you can go beat everyone at the Battle Tower.
2. Get the DLC. This is completely optional. The DLC provides a nice storyline and an opportunity to catch many new Pokémon, as well as Urshifu. While the Battles are led than entertaining, it is still enough to keep you occupied.
3. Try and complete the Pokédex. While this is impossible without Nintendo Online, it can still give you something to do. For me, I love to try and complete the Pokédex as it is fun for me.
4. Breed for something. This always is fun for me. Trying to breed a Pokémon fit for battle has always been interesting. Getting IVs, EVs, etc. Is good fun. Frustrating, but fun.
5. Shiny Hunt. Want something that will take a good amount of time? Shiny Hunting is your answer. The satisfaction is great.
You can also do Max Raids to spend your time on.
TD;LR: You didn't miss anything in the storyline, but there is much you can do. Hope this helps!