I wouldn't want to take day-trips to places 2 hours away.
I've been to Portugal twice, but that was long ago when I had somewhat limited vacation time and didn't do the sort of slow travel you (and I) now prefer. And I don't travel to Europe in the winter (average highs do not adequately convey how chilly it can be at 9 AM or 6 PM), so I don't have a strong basis for contributing thoughts here. However, I'm looking at those 3 nights in Coimbra and Aveiro, and thinking they might be too long unless you have side-trips planned. Aveiro isn't far from Porto; there's both bus and rail service, taking no more than 1-1/4 hours (according to Google Maps). Aside from reducing the number of hotel changes, I like the freedom of longer stays in fewer places when I have a list of potential day-trips to draw on. I can look at the weather forecast and try to suit my activities to the expected conditions each day.
I enjoyed the market (still on Thursday, I think) in Guimaraes, another town that is probably a viable day-trip from Porto; I'm guessing it's year-round but haven't verified that. There's also Braga, but I haven't been there.
Viseu's off the American-tourist route, but it's east of Aveiro so probably a bridge too far if one is staying in Porto and day-tripping to Aveiro. If you stick with the plan for 3 nights in Aveiro, you might read up on Viseu and see whether it would interest you.
Evora's beyond my usual day-trip limit. I'd try very hard to find an additional point of interest in that area (with bus service available) to justify 2 or 3 nights in that part of Portugal. I haven't been there myself, so I have no suggestions to contribute.
On my second trip to Portugal I took advantage of a bus tour from Lisbon that got me to several smaller towns north of the city in a single day. I know one stop was Batalha (for the abbey). Obidos may also have been included; it's a very touristy walled town in the summer. It might be dead in February, but the wall would still be there. Of course, such tours might not run frequently--or at all--in February. I don't know why I'm happy to spend so much time on a bus tour if it gets me to at least 3 small towns but am unwilling to do 4-hour round-trips to a single destination, but that's how I feel.
What I see missing from your mid-winter trip is the lovely island of Madeira, but it would require a flight from Amsterdam (if available) and back to the mainland. Madeira is drop-dead gorgeous and considerably warmer than Lisbon. A slow traveler could occupy herself for a week there, I think. There are great walking opportunities along the (flat) irrigation canals (levadas), and there's bus service to most of the places a tourist would want to go. Although Madeira is unlikely to be chilly at any time of the year, you're not guaranteed to have dry weather there, especially outside of summer.