Hi I know this post is a bit old but I hope your opuntia came back to life.
I notice that you are in a single glazed flat
Have you considered that the window area may be too cold during winter but perfect in summer.
Cold + less light may equate to your poor growth.
You can get cheap temp / humidity monitors on eBay for about £7 which runs on the Tuya app. If the area falls below your set parameters it will alert you and it also lets out a little beep.
There's also Tuya plant monitors that poke into the soil that will alert you via the Tuya app.
I'm having issues with mine , moved it away from the window and put it under a lamp on a timer and it's starting to grow fast.
Th
