Plant seems to be too deep/low inside of plastic pot - you didn't put in enough mix. I see that happening a lot and do not understand why ppl are doing it. Why not add another handful of mix? It can't be cost - for that extra amount it is negligible.
Having plant so deep - and I believe your mix stays wet for too long - is creating stagnant air around the base. On top of that, plants with bigger leaves restrict air flow even more. Then, pot is inside of another, fitting very tight and also deep again. There can't be any decent air circulation around the base of plant.
Watering by soaking is just fine, but mix should be well draining. Yours may be but I don't think it dries up between watering. I do not see enough perlite there for mix to be 50/50 - but photos could be misleading. How do you determine when to water again? Just touching top of soil is misleading, especially if you soak from the bottom. You need to water when the mix is almost completely dry deep in the pot. Use bamboo skewer or chopstick to check.
It is good idea to reduce watering in winter, but that will also depend on temps plant is kept in. Temps could be as low as just below 50*F, and the mix is kept dry almost all the time. If it is warm, watering may be needed more often, but only when needed. Water from top, maybe you can control it better. I would recommend re-doing the mix and using more of it in pots. If you need a saucer, get one. Even plastic top from a container (like a cottage cheese) will do, and much better than tight fitting pot.

As far as planting too deep, not everyone knows these things and I'm sure other beginners like myself aren't aware of the impacts like you've explained, sometimes we learn by mistakes. I usually stick my pinky a few inches into the soil to check the dryness, or feel the soil around the drainage hole. I know overwatering is more of a concern. But the general air circulation point you made was definitely something I overlooked and explained the way you did, makes sense that it may be the reason my plant is stay moist and molding. I will repot with some extra perlite, and give it a proper saucer.

Thank you for the well worded advice.

When I first saw this I thought the same thing as Rina. That doesn't look like 50/50 perlite cactus mix;), but like she said pics can be deceiving:). Adding more perlite should help. Along with planting higher up. I see this a lot, planting too low. A woman who lives by me is guilty of this. All her plants are planted way to low, and her plants suffer from it. Moisture gets stuck and surface of the soil is constantly dark because the pot casts a shadow. If the plant is really small and planted to low the plant will become etiolated because of that shadow.

Honestly your plant looks good and this is an easy fix. Just add more perlite and plant higher up;). It's all a learning lesson. Goodluck;).

I just took this pic. This is exactly 50/50 perlite and cactus mix;).

I am guilty of not measuring the mix, and probably should be doing so based on that pic (lol) , you are both right the mix this plant is in could definitely use some more perlite. And I'll be sure to remove old soil, and wait a while before watering. Thank you!

You would be better off planting directly into the terracotta pot which also allows evaporation of moisture through the sides.

Nice looking plant though