Hi @elisajordan , Ill give to you straight because I know there is a lot of chatter on this topic across the community forums. There is NO solution to the problem you are describing. I am a Shopify App developer and I can attest to the fact that there is no app or themes which will accomodate what you are describing.
What I described was an app that can help you promote a discount “codes” (not automatic discounts) in the collection page, product page, and cart page. Now I know you are looking for an app or theme modification that can acomodate “automatic discounts”, unfortunately this does not exist (I am investigating intensily a way to get this done dynamically for every theme but it is not easy).
So if you are open to creating a discount CODE (instead of automatic discount) for the collection you have, then you will be able to design a badge with the percentage discount on every eligible product in the collection page using Codeboost. Additionally you will be able to do this privately to customers (which automatic discounts do not allow). This means that 1 customer might see eligible products while another might not, this can help you increase loyalty for your most VIP customers.
Happy to answer any questions you might have about the app if you are willing to try it 