Set Category wise Discount in Magento2

You can do this by catalog price rule for specific categories. For adding the rule please refer this:

https://www.magestore.com/magento-2-tutorial/how-to-create-and-manage-catalog-price-rules-in-magento-2/

Note: You asked that customer won't be informed that the rule is applied -->the discounted price will be shown on the product detail page of the products of those specific categories.

You can do it by creating either catalog price rule

1) you can set either condition OR actions in above rules

2) For catalog price rule using condition method just go to condition tab -> click on + sign button -> click on drop-down -> select category and select your category from just right of the textbox and save this catalog price rule

NOTE: You have to create separate catalog price rule for each category due to your discount amount