car hire vs car rental

Yes, the rentals are crazy expensive right now. But, they might come down by September. If they do come down, it’s quite easy to drive in Portugal as long as you aren’t in Lisbon.

I really don’t know if Uber will go across the border into Spain but I’m guessing it would be even more expensive.

FYI - Porto is really too far for a day trip, but 2-3 nights would be doable and easy on the train.

I can't see this saving you any money.
Not sure how Uber drivers work but sounds like this would be an off the clock type thing that Uber would not be aware of so would need to make it worth their while to miss 2 days of regular work.
Best would be to email a car service in Portugal that offers airport rides or similar (a small local company not Uber) and ask them what such a service would cost.

If you wanted to hire a driver for 3-4 hours that would be one thing but to hire a driver for multiple days to travel long distances including over a border ; you would have to be well in excess of $200 ; probably multiples of it.

As mentioned car rentals are really high right now for a few reasons which may start to see improvement in Sept.
In case you were not aware Uber costs are sky high now too with surge pricing often in affect due to labor shortages and drivers paying much more for gas, wear and tear, etc... Those high costs are more likely to last through Sept.

If you book the rental now make sure you can easily cancel free of charge once you get a better rate.