Train bus or car Galicia and Portugal

Trains are fine from Madrid to Santiago (direct, 5.5 hours), if you're going straight there; but not great on the border crossing southbound from there (but there are one or two direct buses per day); and the only train from Lisbon to Madrid is the overnight train. Looking Up Train Schedules and Routes Online gives you the Deutsche Bahn train schedule link and tips for using it.

Renting a car is great if you want to make additional stops that you haven't named, but not ideal for covering long distances quickly.

Budget flights are likely options for any/all of these legs; see www.skyscanner.com. A Coruna airport could also be an alternative to Santiago de Compostela airport.

Planning to stop in
Segovia Avila Salamanca Santiago A Coruña Vigo Porto Coimbra Lisboa etc

Hi,
Even in three weeks not a lot of room for etc :-)
You left out Madrid: intentionally? It changes things a lot if you don't need/want to visit Madrid.

All of those destinations are served by train, but to "set your own schedule," I would probably drive.

As Laura said, if you’re only going to large or mid-sized cities, trains should suffice. You also wouldn’t have to pay for parking. Parking can cost between €15-€20 per day in cities. Where a rental car works better is when you’re stopping at places not served, or regularly served, by public transportation. In norther Spain for example, many castles, cathedral beach, Cabo Ortegal, O Cebreiro, Picos de Europa park, Muxia, church of San Salvador in Vilar de Donas, etc.

You mentioned Vigo. Any particular reason for including it? There are a lot of interesting places in Galicia; I didn't find Vigo to be one of them.

We fly in and out of Madrid ( October 9 to the 30th) and although we have visited before plan to stay there 2 full days and nights before renting the car and heading to Galicia. Will also be there the day before we fly back home.
No particular reason for Vigo; its on the way from Santiago to Porto.
Route, stops and sleepovers advise welcome on this Madrid-Santiago-Lisboa-Madrid trip. Another option could be driving up to Santiago and returning the car there. Then train and bus in Portugal and fly back to Madrid from Lisboa.
Advise appreciated. Thank you!

I liked Pontevedra. The small coastal villages of Cambados and Combarra are touristy but attractive. Well, perhaps not so touristy in October; they might be totally dead. The border town of Tui has a picturesque historic area. Closer to A Coruna is Betanzos, which seemed non-touristy and provided several hours of pleasant wandering through its historic center.

I needed a mix of trains and buses during my time in northern Spain.

Check historical weather data. I'm not sure you'll see much sun in Galicia in the fall.

We spent a few hours in Zamora (north of Salamanca). We couldn’t see the churches because it was Saturday evening and there were several weddings, but it was fun to observe people arriving for the weddings.

We really enjoyed the Roman gold mines near Ponferrada. We stayed a few days along the Sil River gorge, enjoyed hiking to churches, beautiful scenery. Don’t miss Hercules Tower in A Caruna. Also enjoyed Celtic sites along the coast west of Santiago.

I recommend getting a car. You can’t see these rural sites without one.

I like your option of driving Madrid - > Galicia then continuing to Portugal by public transport and flying back to Madrid. I'd allocate 2 nights for Madrid, 10 for Madrid-Galicia Road trip, and 8 for Porto-Coimbra-Lisbon.

About 10 years ago we traveled with our bikes and a rental car Madrid-Picos-NW Coast. We stayed in off the beaten track places for specific bike rides near the ocean and nearby hills. Without digging in the family archives, I remember the cave painting museum (not the real paintings) and a local zoo. Beautiful ocean views as we rode our bikes. After the foothills of the mountains, it is desert. It was a long day trip back to Madrid. My suggestion would be to get to a more central location before renting a car. Utilize train or flight to get back to Madrid.