I got this cactus a couple of years ago but in the last few months I have noticed the bottom half getting brownish and soft. I water the cactus about once every 6-8 days and it gets a lot of direct and indirect sunlight as it sits on a SE facing windowsill. I have fertilized the cactus once with a cactus and succulent fertilizer. I also repotted it and the roots looked healthy (a white creamy color). I live in the Baltic.
It's not clear from the photos, but cactus often have corking or browning of the lower stem near the soil line. It is not normally soft, so that is a concern.
If you could take a picture or two with better light on the lower stem that would be helpful in determining if you have normal corking or if something else is going on.
Your cactus is a Echinocactus grusonii which are fairly robust. Unfortunately, it won't flower until it gets bigger than your head.
Do you know why the lower spines look stained red? I've never seen anything like that.
I caution you to water when the soil is mostly dry rather than on a time table. When it's cold and/or humid the time between waterings should be greater.
It's a good looking plant, I hope it's just corking.