You'll still want to complete other collections (if you haven't yet), but with so many interesting cards to get on Mythical Island, I wouldn't sleep on the Mew packs for now.
Instead, I want to fill my deck with as much incidental interaction as I can, and thats pretty easy with the likes of Aether Meltdown, Deadlock Trap, and Confiscation Coup
Offering card draw, removal, and a somewhat expensive, win condition, Vraska, Betrayals Sting does it all
And in Expanded in particular this is very easy to pull off with a card, Hypno-Toxic Laser (PLS 123/135), which poisons your opponent's Pokemon and puts them to sleep on the flip
Then you can Zombify that many critters from your graveyard to your battlefield, so this is great after a mass removal spell or in a deck with a milling or graveyard centric theme which we've looked at earlier with Beseech the Mirror that works in so many decks and infrastructure