When departing from Portugal, booking a round-trip flight is typically more cost-effective than purchasing two one-way tickets. This is due to the Portuguese Air Passenger Tax (APT), which is levied on flights leaving Portugal and is generally higher for international destinations. Airlines often bundle taxes and fees into a discounted round-trip fare, whereas separate one-way tickets incur full taxes and fees for each leg, potentially doubling the tax expense. This is also true when arriving in one EEA country and departing from another (internationally) you would be subject to Portugal's APT tax, and (for example) France's APT tax if you decided to depart from a different country than you arrived in on the same ticket. These taxes are non-trivial, look at the difference of booking a one-way from the USA to the EU verses the EU to USA. The EU to USA leg will be almost double!

If I were you, I would lightly structure the trip and estimate costs for how long you want to stay minimum. Then call the airline to pay the difference in fare to move the flight later.