Welcome to the Seller Community.
We definitely see how this could be a benefit for your business model. Currently we do not have a way for an automatic discount rule to apply to multiple variations of an item. You would have to create each item quantity/price variation as a separate line item. For example, the $6 or 4 for 20 would be one product and then the $9 or 3 for 25 would be another.
Our product teams are constantly looking through our seller's feedback to make necessary improvements. So we will be sure to let you know if we have any updates.
Please let me know if you have any other questions by replying to this thread.