I am new to this forum and need some second opinions. I have what I believe is a Mammillaria Backebergiana named Spiky, and he is my first cactus that I got a year ago in March. Unfortunately due to the soil that I was using and overwatering, he started to get what appeared to be root rot on one side. I cut him roughly 9 days ago in order to save him from the rot, and wanted a second opinion as to whether he is healing properly. He has some small brown/shiny spots on the callus on the same side the rot was that I'm not sure whether it means that I need to cut more off.
Attached below is what he looked like originally before I cut him and the part that I believe was beginning to rot (I cut more than that off).
This is the cutting a week later. It has dried considerably and I think I see a stub of a new root growing. You can see the brown/shiny spots at the top and right on the forming callus.
I have since picked up Sulfur powder, and was wondering if I need to cut higher up. If he's healing well and the spots are a caution, what else should I be watching out for? I really want this guy to pull through. This is my first time posting in a forum, so my apologies if I've violated any forum etiquette. Thank you in advance for your help, I really appreciate it!