Is my Portugal itinerary too ambitious?

I won't comment on your itinerary because we all have our own pace and priorities. We spent 5 days in Lisbon and 3 in Porto plus a day driving between the two with a couple of stops and thought that worked well.. We did a daytrip to Sintra and wished we'd spent 2 days.

Where are you staying in Porto? Parking was €50/ night near where we stayed in Ribeiro. Consider not getting your car until your ready to leave. We paid €14 for Uber to the airport.

That is a lot of driving to/from Salema for a small payoff in my opinion. I am not sure you can fit in Sintra in the same day. I would cut out Salema and add extra night in Lisbon. Even then, I would find your itinerary to be too packed for my tastes; I am usually not a fan of one night stays. Good luck.

Porto>Coimbra>Lisbon are linked by high speed train.

Evora can easily be done as a day trip from Lisbon via frequent buses or trains.

So, if willing to remove Salema and Nazare, you can reduce the 1 night stops and not even need a car.
If you require a beach, can always do a day trip to Cascais from Lisbon via commuter train.

We went to Algarve and enjoyed it very much. But it is way too much trouble for one night. We stayed 3 nights.

You do not want a car in Porto.

Driving was more difficult than we expected so the idea of having an itinerary you could do by train is something to consider.

I really appreciate all your responses! They are very informative, so keep them coming please. :)

We did an overnight in Sintra and were glad of it. You're not going at the thick of tourist season, but expect it to be crowded (we went in late September, and there were LOTS of people in Portugal) If you think you want to see more than two castles, you will have to rush. We got into Sintra before noon, and went to the National Palace and Quinta without reservations (did have to stand in line in Quinta, but not too long). The next day we saw Pena Park terraces, Moorish Castle and Monserrate, and that was about all we could fit in.

Just curious, why Salema?
Want to visit the Algarve region?
Want to visit a beach town?
Because Rick Steves mentions it?

Why did you skip the amazing Douro River Valley? It is the most scenic river valley in Europe.

A one night stop on occasion is fine, but I think 3 in a row is too fast of a pace.  I agree that the Douro Valley is worth a stop.  What are your interests and priorities?  

Based on all the great comments, we've decided to only stay in Porto and Lisbon and take day trips from each city. The Duoro Valley would be wonderful, as well as the southern areas, but unfortunately 8 days are not enough. What we need now are recommendations on lodging in Porto and Lisbon please.

Way too ambitious. If it were me I would divide your time between porto and Lisbon (with possible day trips to Sintra/Cascais and Evora. With the itinerary above you will spend most of your time driving and checking in/out of hotels.

We have our flights booked from U.S. to Porto, lodging in Porto 3-nights at Uba Heritage & Wine (as recommended); lodging booked for 5-nights in Lisbon at Airbnb in Principe Real (just north of Bairro Alto). Now, how to find guided day trips from Porto and guided day trips from Lisbon. Also, how do we get from Porto to Lisbon without a car?

Take the train. it's about 3.5 hours on the faster AP trains. Book on Combois Portugal online at https://www.cp.pt/passageiros/en

We took the Living Tours day trip from Porto to the Douro Valley and had a great day. Booked through Trip Advisor.