I’ve taken the bus from Santiago to Porto and then onto Fatima. Afterwards, we traveled to
Lisbon by bus to finish that journey. It is simple, comfortable, and much cheaper than the one-way rental cost of a car plus gas and tolls. Also, realize that you want to rent in one country and return it in another - that may increase the one way fee or be outright prohibited. You do not need a car in any of those cities (Porto, Fatima, Lisbon) nor is it a particularly interesting or beautiful route that would be better seen through a car window versus a bus window.
As for the IDP, it’s required by law. Just because a rental company doesn’t ask for it doesn’t mean you as a guest in another country should be emboldened to break it. Plus I’m the event of an accident or police stop, you’ll wish you had it. Plus some credit card companies will not honor your insurance coverage of you make an international claim if you do not have it.
If you are still set on renting, I’ve used several different car companies in Europe and they all seemed equivalent to me. I make my decision based on price, location of agency, and car availability.