I am attempting to do a "sale on sale" promotion for one of my clients. They would like to add a discount on top of items that are already on sale. Example: "take an extra 30% off sale items." Now, I know that it works through the cart promotions, but the customer wants it done through the catalog promotions. If an item has a standard price of $75, the client brought the price down to $50 using the "special price" option on the product. Now they want to bring it down to $25 through the promotion, but they want the website to reflect that new price. So they want the original price of $75 to be struck through and now show that it is a $25 item instead of a $50 item. How can I make Magento do this?
You should be able to get this done using Catalog price rules https://docs.magento.com/m2/ce/user_guide/marketing/price-rules-catalog.html
You should be able to define a condition to apply to all discounted products however you may need to tweak your percentages as these values work based on the original price and not the already discounted value you have defined (eg : discount perc = sale% + additional%)