There should be service in the afternoons too on the postbus Brig/Domo...why the timetable is not showing it I don't know. A lot of people live in Simplon Dorf or Gondo and work in Brig.

Furkadampfbahn - indeed an amazing thing! Can't say I have done it because I haven't but it is a ride I really want to do. Goes between Oberwald in the Valais and Realp in Uri. And is 100% steam railway. Very seasonal though. In June 2020 it will run on Fri, Sat and Sun from 13 June-6 July then daily. So I don't think this fits with your other travel plans?

Centovalli -- far better to take the train than to drive! Driving that road is a nightmare but the scenery is beautiful. If you are on the train you can look at it 100% of the time. On the road you are too busy dodging Italian lorries to appreciate the views.