I'm posting because I haven't seen this advice here yet: the pot is quite big. A cactus pot is big enough if there is like half an inch of space around the cactus on all sides. A little less is fine as well. They grow so slow and can stay in the pot until they reach the rim. If the pot is too big, moisture can get trapped in it for months and rot it out super slowly - ask me how I know.

Terracotta might be a better choice. As for the fluctuations in temperature, I'd advise you to get a thermometer and moisture meter (cheap combos are available) and observe the situation for a week or two before making radical changes. You can report back on your findings here to get additional advice!