Private driver San Sebastian to Porto

In the south there's bus service from Faro to Seville if you're heading down there. (Getting back to Madrid from southern Portugal would be more challenging.) That won't be as time-consuming as what you're facing between San Sebastian and Porto.

I have never driven in Europe or paid for a private inter-city transfer, so I can't help you there beyond noting that ViaMichelin estimates the drive you're contemplating at almost 8 hours, so you'll not only be paying for close to a 20-hour round trip (including meal and comfort breaks), but presumably also for the driver to spend a night in a hotel. I would expect that to be very costly indeed, and a one-way car-rental might be cheaper.

Do you have a quote for the transfer?

The transfer to porto would be one way. Approximately 7-8 hours.
Cost about $750-800 for two people.
Flight would be around 300 for two. Plus transfers, and probably take same amount of time. But more hectic and stressful.
I had not budgeted for this high cost transfer, however, if we can treat it like a tour, and it makes our day more enjoyable, is it worth the price? I know you can’t answer that for us. I guess I am trying to justify it.

Great info on the bus to Seville from Faro.

Thank you for taking time to respond.

And I will continue to research the car rental option. It might prove more affordable.

What I meant was that it's very unlikely your driver will have a fare back to San Sebastian from Porto, so he's going to be charging you for two days' worth of his time. That quote doesn't sound bad to me, given the time involved. It's a shame the basic drive takes so long; there won't really be time for sightseeing pauses along the way. The Basque Country has some very nice scenery, but I can't tell you what you'll see on this particular drive. What you might do is go to ViaMichelin.com, ask for a routing between the two cities, then zoom in on the map and follow the route. Any really scenic roads will be outlined in green, but you have to zoom way in to see that.

There's a 10-1/4 hour rail connection between San Sebastian and the very interesting Santiago de Compostela in Galicia, which isn't all that far from the Portuguese border. It's an hellaciously long trip, obviously, but there's only one transfer required (in Vitoria-Gasteiz), which seems almost miraculous. The departure is at 12:13 PM; the arrival, at 10:59 PM.

What are the other places you are visiting in your trip? It looks like you could reshuffle some destinations to make things work better. For example, Bilbao to Porto is much easier than S. Sebastián to Porto, thanks to direct flights.

Southern Portugal to Spain will not be as hard, since there are buses taking less than 4 hours from the Algarve to Seville and flights from Faro to Madrid.

But more hectic and stressful.

Not sure why you would say this. Having just come back from 60 days in Spain I would fly that leg over sitting in a car for 8 plus hours any day.

There are many interesting places that you could use to break up the train journey and spread it over a few days. But if you want to do it in one day then flying is the only realistic option.

A private driver is not a machine, for your transfer to be safe they will need rest stops and you will probably need to pay for a night's accommodation for your driver in Porto before they head back to San Sebastián.

If you really want to go by car and don't want to drive yourself have a look at BlaBlaCar which is a ride sharing service used in Spain. You might be lucky and be able to pick up a ride with someone.

if we can treat it like a tour, and it makes our day more enjoyable,
is it worth the price?

In my mind a tour means you make stops and go out and look at things along the way. This will be 7-8 hours of sitting in a car seat, with a stop for lunch somewhere. Not my cup of tea.

How much time do you have between the towns? Getting from one country to another is easy, you take the train from Vigo to Porto. So the question is how you get from San Sebastian/Donostia to Vigo. And you should really make a stop or two along the way, the Spanish atlantic coast is an underrated part of the country in my opinion.

Thank you so much.
This is the exact input I was hoping for.
I am learning a lot already through your suggestions.
I have not booked anything but the flight so my itinerary is flexible.

Not sure how long you’re giving to get to Porto, but there are dozens of worthwhile towns and sights in Spain you could stop at. With so many fantastic places it would be a shame to simply bypass them and go directly from point A to B. Same thing applies for going from Portugal back to Madrid, but I don’t know your itinerary.

For a trip later this year we’re renting and returning a car in Madrid. The middle of the trip is Portugal.

Edited: maybe it would have been better to say specific itinerary, I.e., how much time you have devoted to each segment of your trip; x days Barcelona, x days Madrid, etc.

This is a 3-week trip beginning in Barcelona, ending in Madrid and with San Sebastian, Porto, one assumes Lisbon and "southern Portugal" in the middle. That doesn't really leave much time for stopping over in additional Spanish cities.

Researching deeper I think flying is our best option.
Wondering if the flights with a stop in Madrid require a change in planes or is it just some passengers departing and others boarding?

I know Bilbao has non stop but the only flights I am finding now depart at 7:00 in the evening for our dates in September.

Wondering if the flights with a stop in Madrid require a change in planes

Yes. Change planes.

I know Bilbao has non stop but the only flights I am finding now depart at 7:00 in the evening for our dates in September.

Is it a problem? With the time difference, you will land in Lisbon at 7.15 and be in town for dinner.