Magento – Set Category wise Discount in Magento2

Yes, just give them the discount divided by quantity step. If you want to give $2 discount every 3 items then you would set a Fixed Amount Discount of 0.6667 ($2 / 3) then set the Discount Qty Step to 3. Every 3rd item will result in 0.6667 * 3 discount, or $2.

The individual sales rules are possible, so you can create a cart rule that will give a percentage for any order containing items bought from a given category. You can then create other rules for other categories. So each of them could cumulatively give e.g. 2% off the order.

However, those rules are not aware of each other. So they do not know what the current discount is (from the other rules) and cannot then have the 'if' to decide whether a further discount is possible.

As for the product specific discount, you can do that one with a rule or a just a special price on the product.

If you want to do this with a module then the easy bit is coding the rules that you want. Quite a few other changes are needed. There may be a need to update templates (to show your extra line item), some xml for 'totals_sort' order - it goes on.

I also feel that your original sales idea might actually be confusing for customers. Sometimes the spend X get Y rules need to be easy for people to understand.