These are the options I'm aware of.
1: Set it up on each card (as you know is more time consuming)
What would probably be the best way:
2: Use Personalized Data Permissions (PDP) policies to make it so when an external user looks at a card they only see it where the data is external_user_field = True. You can setup one card/set of cards for both sets of users and when the external users look at the card they only see it from their perspective.
I agree! This would be huge if admins could push-out page-level filters. Also if they could have default values selected on card page-filters so that the page is still filtered a certain way by default but have the option of changing a page filter temporarily.