For the few very cold days to assist in a fast warm-up have you considered a block heater?
https://www.subarupartspros.com/part...ck-heater.html
I used to option my new cars with a block heater but have not needed to in the past 20 years. Reasoning when I will really need it there would be nothing to plug it in(winter camping anyone?) A good strong battery is more important when it gets that cold.