Hi, not sure if you have a basement or not, Japan maybe a single room on a highrise. However maybe you do. Others that do have basements are insulated for sure from temp swings.... But to find the ideal spot as well, this is my experience
Bought one of these, not cheap for sure, used its eight (8) sensors to find the ideal spot of RH and temp (1 deg F fluctuation year round) in the house, been happy since. Used it for other purposes, as in fridges, deep freezers, outside now, and also helping others find the solution to where to age wine in their house. You may find it useful as well. I didn't have to make a cooler, fridge, or otherwise cellar. The location as expected was in the basement, but not in a spot I expected. This item really cut down time, and energy, and has helped countless club members with the same issue, finding and using a spot for aging wines/beers, or otherwise monitoring fridge/freezer temps. Hope that helps.
https://www.amazon.com/gp/product/B07CLHH7Q2
I also know that folks often make mead in the Fall (harvests) and cooler temps, bottle and age in the winter while inside more than outside, as well they can and do make insulated cooler/aging boxes so temps don't fluctuate as much. You sound like you have a very big delta in temps per day or seasonally.