Hi Fede,
Please refer to these blogs for Tax and Discounts on Quotes / Orders in Dynamics 365
1. if there is the possibility to have a % of discount per each line of a quote/opportunity
Yes, we will create Discount Lists.
Please refer to https://docs.microsoft.com/en-us/dynamics365/sales-enterprise/set-up-discount-list
We can set different discount percentages for different quantity intervals, and apply the Discount List to the Price List Item, so that we can achieve a percentage discount for each row.
2. if there is the possibility to check line by line if an item is taxable or not
Unfortunately, Sales does not have OOTB function to automatically calculate Tax. We need to develop Javascript or plugin to achieve it.
The Product entity has an OOTB field "Taxable".
We can display it on the Quote Line form through Quick View Form or calculated fields or Javascript.
When using Javascript or calculating fields, we need to first create a Two option field for the Quote Line to represent "Taxable", which is not needed with the Quick View Form.
Javascript can be triggered when "Existing Product" is changed, and Quick View Form and calculated fields are only triggered when the form is saved.
In addition, create a custom field "Base Tax Rate" for the Product entity and display it on the Quote Line form, just like "Taxable".
Create a Javascript onChange event for the Quote Line field "Quantity", and automatically update the field "Tax" according to the "Quantity" and "Base Tax Rate".
In this way, we can automatically calculate the tax for each line based on the Quantity of product.