Northern Portugal is not flat, so unless you're riding in river valleys, you'll usually have lots of hills. Fortunately, the Douro valley is lovely and was my favorite part of Portugal for touring. We were riding east to west, ending in Porto. I don't recall how much traffic there was, to be honest.
The Minho/Miņo river forms the westernmost border between Galicia and Portugal. For most of the length where the river is the border (about 80 km) there is a nice paved bike trail along the river on the Portuguese side. I rode that west to east, and then continued on roads upriver in Spain, where the river was entirely flowing within Spain. Those Spanish roads were particularly nice, as the scenery was lovely and there was very little traffic.