Ramping up difficulty

Like if you either need a 9+ on 2D6s (an easier version would be 7+) on your dice... I'm not sure are you using "dice pooling" as a mechanic or is it part of your "quota" mechanic?

If you are dice pooling you can vary the SUM(dice) to be higher than the average "7" like instead of removing dice that are only "8+" you remove "6+" on every two dice and count the remaining dice and score "+1" Point for each remaining die.

Right now I don't know how your "quotas" work with the dice. So it's a bit hard to answer... Because there are so many possibilities.

If instead it is part of your "quota" mechanic and it becomes more of a "worker placement" type of situation where you have to MATCH dice to values to cards and the winner of a card has the more amount of dice on that card... Well then to make it harder ... Instead of "amount of dice" make it about "point matching".

What do I mean? Let me explain: If a card requires a 2-3-4 or 5. If you roll a "3" & "5" you have "8" points. Your opponent cannot beat you even if he rolls BOTH "2" & "4" (because that would mean only "6" points). So you would win that card...

This is a bit of a mix of worker placement and dice pooling too. Sort of a combination of both. The last explanation makes the game intrinsically more difficult because of the added complexity of a "points" layer.

And since it's a CO-OP it could be Players vs. AI Player...

Again hard to give you accurate information with so limited amount of explanation from your end. Best of luck(?!) with your design. Cheers!