Also, I've always considered that the thermal barriers for car batteries was intended to limit the engine compartment heat from getting to the battery in hot weather and/or countries with higher ambient temperatures than UK.
One thing that might assist you in working out what is happening, is a small plug into "cig lighter" ie 12V Power socket, voltmeter, I've had one plugged into the socket on my wife's August 2015 VW Polo 1.2TSI 110PS 6MT for many years - it lets you get a better understanding of how the smart charging is working, and hopefully allows you to spot a dying battery before it lets you down.
Now, that 2015 Polo is coming up for 52,000 miles and it's original Exide EFB is still fit for use, okay it is weakening slowly (as checked at every annual service by me using a CTEK Battery Tester), but if I let the "do it's thing" with auto Stop/Start, the car's engine will cut out after driving 0.5 mile in winter temperatures - as long as I've kept the cabin fan and rear window heater AND seat heaters off.
This car tends to be garaged overnight though, essential to avoid lots of window clearing of frost and snow off car!
Edit:- I like the idea of having a battery monitor that you can view remotely though!
My older daughter's 2019 Leon Cupra also has an EFB - SEAT being a bit tight/cheap there I think, but her car is also on its original battery and I test it at annual services and it seems to be in good condition yet!
