Hi, mandywox, Welcome to the forum! There is a good sized group of forum members who have started in Lisbon. Though there are some naysayers, who didn't like the pre-Porto section, I'd say that most of us really enjoyed that part. Take a look at the guides made by forum members, one for Lisbon to Porto and the other from Porto to Santiago. They are both in the Resources section and there is a lot of good information in them and in several other resources forum members have posted.
https://www.caminodesantiago.me/community/resources/categories/camino-portugues.40/
As of May of last year, there is now a pilgrim albergue in Alpriate, about 20-22 km from the Lisbon cathedral. It is a great first day stop because it gives you time to dawdle as you wind your way through old Lisbon, the old port area, and the new Expo site with its river walk, aquarium, etc etc. With 31 days, you probably have a lot more than you need, but that would give you the chance to take days off for visiting, etc. I think the average from Lisbon to Santiago is probably about 25 days (two weeks to Porto, and 10 or 11 to Santiago).
Take a look at Maggie's Lisbon-Santiago blog, it's very complete.
https://magwood.me/camino-portuguese/
Between Lisbon and Porto, there are several nice places where extra time to visit could easily be used (Tomar with its monastery/castle; the Roman ruins at Conímbriga; the University city of Coimbra).
Some of the concerns in the early stages have focused on the asphalt/highway part of the Caminho. The Via Lusitana (the Lisbon pilgrim association, who runs the albergue in Alpriate) has done a lot of work to move the route off-road, and little by little this is happening. There is a definite increase in the number of people starting in Lisbon, so if you plan to start in spring or summer, you are likely to have company along the way.
Bom caminho, happy to help with questions, Laurie