Increase the cost for cards from 4 to 8. That would make trading in your scoops for cards more a losing proposition thus encouraging people to hoard scoops in the open and try to protect them as much as they can using shields throughout the week.

Another idea would be to make it where you need to trade in cards in specific combinations in order to get specific cards. To increase the combination let’s increase the costs of cards to 5 scoops per card.

You could make the relatively weak cards like destroyer, peeping tom, even thief require 5 cards of one color. The more powerful cards like modests and shields might require specific combinations of cards. Really powerful cards like bumholes might require 1 of each color forcing people wanting them to compete in all aspects of the game not just raiding.

That way you can still reward raiders but by limiting the options for purchasing cards via vanilla you reduce the desire to turtle up with modests and shields.