Are Braga, Guimarães or Coimbra worth adding?
Yes, but selectively. Braga and Guimarães add northern depth; Coimbra can break the Lisbon–Porto line; none should be added just because the itinerary looks too empty.
Answer
Portugal’s smaller cities are often where the trip gets less performative. Braga and Guimarães work well from Porto if you want history, squares, churches and a slower northern layer. Coimbra can make sense between Lisbon and Porto, especially for travellers who like university cities and a softer pause between the big names. The catch is that these stops are route seasoning, not automatically the route itself. Add one because it changes the texture of the trip. Add all of them to a short itinerary and you may spend more time managing luggage than noticing the place.