All indirect tax systems I know of, such as Sales Tax or VAT, require to apply the tax on the net amount after application of the discount (gross minus discounts). Therefore your module has a functional flaw.

Applying global discounts is easy as long as you only have 1 applicable tax rate for all sales order lines, but it gets complicated, if you have different tax rates applicable to different sales order lines. In the latter case, you have to apply the discount to the subtotal for each tax rate in the case of a global percentage discount and in case of a global discount amount you have to split and proportionally apply the discount amount according to the subtotals per tax rate. And you have to consider the handling tax included or excluded.

Summary: Applying global discounts is not easy from a legal point of view and therefore an implementation must be undertaken carefully. This is not Odoo-related.

Amendment:

I don't know any modules, but by following my own advise I did not look around anyway. When there is no other way than applying a global discount, I do it manually. If the global discount is supposed to be by percentage, I apply this percentage discount to each order line. If it is a discount amount, I add one or several discount order lines for each tax rate manually. You may define discount products for that purpose.

Hello Ermin, 

could you please provide me with one module if it has the requirement that I need, because after searching through couple of modules in the store, all of the modules  that I've found, calculate the TAX based on the subtotal not on the amount after discount.

actually for my database I'm considering the Tax as VAT, so it's 5% VAT all the way, and it needs to be applied on the discounted amount, so if there is a way to do it, since I have only 1 tax rate ( VAT)  please let me know,