Not sure if this is generally known or not (I only found out today) but there is a way to see which star and crown cards are missing from your card dex...

1. Tap the 'Binder' toggle on
2. Tap the magnifying glass to search
3. Choose the required star/crown rarity buttons
4. Scroll down and at the bottom of the page you'll see a new 'Card Dex View Options' section appear. Toggle the 'Cards with a rarity of ⭐ or greater' toggle
5. See a list of cards with spaces for missing star/crown cards.

Hope it's helpful for people. Please don't shoot me if it's generally known! 