Maybe they should limit the deck based on the total number of stars instead.

For example, if the total stars allowed is 17, then we can use:
- 4 3-star cards and 1 5-star cards like right now, or
- 3 3-star cards and 2 4-star cards, or
- 3 5-star cards and 2 1-star cards, etc.

It would also be easier to incrementally increase the limit. Going from freely using 1 star cards to 2 star to 3 star, each represented huge leaps in our deck strength. And going to 4 star would also be a similar leap, rendering many of the npc trivial to beat. I know the OP is not suggesting this, but that rule feels needlessly complicated and still feel quite limiting to me. By limiting the total number of stars instead, there would be a lot more variety of decks that we could create, depending on the npc and the rules at play. And also it is easy to incrementally up the player's power level every 30 cards (or whatever) that we collect like last time, just add 1 more star to the limit each time.