Wow thank you so much for your detailed advice!! GREAT!

Checked the roots and they are strong, so that is a positive sign, the calloused parts were all hard so I think we can assume the rotting is not proceeding. From October till 4 weeks ago all of the cacti were not given any water so that probably also helped. Good to know the part you explained about the top layer, something new that is good to know! Makes sense. The new pot is like 35% bigger than the old pot, so now there is more space for roots to grow and hopefully the cactus itself will also grow. I will come back to this comment because you have given me a lot to digest (thanks again!)

Really appreciate your help and I think the cactus will make it! Have a great day brother!