Dinner reservations??

In Sintra we really enjoyed InCommun, and yes, you will need a reservation. https://incomumbyluissantos.pt/

In Coimbra, we were able to get a table without a reservation, but we were there when the restaurant opened and we were there in April, which is slightly less crowded.

Three nights in Coinbra last September and we didn’t need a reservation.

I didn't book anything anywhere last week, including the city center of Lisbon and Porto. Sometimes we had to wait 15-20 min to be seated at a table, but nobody rejected us, and July is certainly busier than September. It depends probably on the number of people in your party. If it's only 2 or 3, you will probably be able to be seated nearly anywhere without reservations. For a larger party, then it might be a good idea to reserve a place, especially on weekends. I've never booked a place ahead of time more than a day ahead, more often than not, if I book a place, I do so the same day. Booking two months early like you did is something I've never done in my life anywhere in Europe.

I have never booked before in Europe either. Just reading how touristy Portugal has become lately. I think for those two cities I will just wing it. Hate to be boxed in by being somewhere at a certain time. There were particular places in Lisbon and Porto that are quite in demand so I am happy I have those reserved.

My husband and I had an extremely tough time finding a restaurant for just the two of us that would take us in Porto, April 2023. When we did find something, we often had to wait at least a half hour. We did find a couple restaurants in Lisbon that would take us, but when we had a specific restaurant in mind, we made reservations, and as we watched folks turned away, we were happy we did. If you want to eat at InCommun in Sintra, I'd be surprised if you could get in without a reservation.

We rarely make reservations in Europe and we do prefer it that way, but there are certain places where reservations are prudent.