I completely wiped my Surf Soho Mk3 router to make sure I got rid of everything related to that firmware roll out as a last resort by factory resetting it.
I then manually uploaded the 8.1.0 FW image (that was the newest one at the time), and did not use InControl to do this, and I did not download the update through the router either. Connected to the router via ethernet cable.
After that all and some other details that you’ll see his post below, all of my problems went away and today it’s working just as well as it did when I got it three years ago.